Advice on Resonance Inside Car

[Guest]

I drove my car down here from England with the hard top on last April. All I noticed was wind noise from the front top seal and I had a pleasant trip. There was also a slight vibration (felt through the steering wheel and seat) as you drove off and went through the gears. This also occurred when the vehicle was stationary as the engine went through 2000 rpm. I just accepted that this is something that the car did and it never bothered me. I have been driving with the hard top off ever since and everything seemed fine.

I put the hard top back on last week and straight away I get this horrible drumming inside the cabin at certain speeds. The car itself does not appear to be vibrating, other than that mentioned above, just resonating. I have been out again this afternoon to see if I can narrow things down a bit. At first I thought it was rpm related, but I am now convinced that is speed related, as it manifests itself around 20 mph in all gears. It makes the car very unpleasant to drive and buzzes the inside of your head!

I changed the diff seal and quill shaft bearing soon after I got the car and that has set me wondering if I did something there. I put the universal joints back in the same position as they were when I undid them and they were not sloppy. The other thing that I know is that I could not get the exhaust pipes to go back through the rear suspension holes in the same way as they were before and they do touch each other. I had a go at fixing that, but failed. My mountings at the holes do not seem to be as per the book. So, I’m hoping that this is the root of the problem, but will have to wait a bit until I get another car off the ramps before I can investigate further.

In the meantime, has anyone else experienced this drumming inside the hardtop and what was the solution?

Don

[StagnJag]

It could quite easily be the quill shaft bearing. Some bearings do not last long - was it a branded make? If it is engine speed and not road speed related, it could be the viscous coupling. It might also be worth checking the diff nose extension for signs of cracking. If something is beginning to move the prop will run out of true and vibrate.

If the exhaust is touching the subframe arms it could also be that. What I have done with mine is used the subframe exhaust mounts purely as a hanger, and used a seperate clamp to joint the exhaust bits together. It gives a bit more scope for adjustment.

Rgds

Dave

[Duncan Skye]

Hi Don,

You have two different things going on here, possibly not related. There is a vibration from something mechanical as Dave has responded to, and with the hardtop, where the concern seems to be speed related, at about 20 MPH. This tends to make me think that you have an aerodynamic problem. For example, if you drive along in your new eurobox and open one window, you'll probably get a resonant buffetting at some speed or other, normally below 40 MPH or so. It will become less apparent with speed where differing flow dynamics (wind roar) and other car and road noises gain prominence and nullify it or drown it out. At its worst, it may be felt as a heavy resonant buffeting, mostly felt in your ears as if there were a huge bird beating its wings inside the car. The effect may be so uncomfortable you have to either open other windows to cancel it out or close the car up to get silence and rely on the blower motor.

To find out if it's something like that flowery description, try a simple check to see if it is an aerodynamic problem by driving along with the effect at its worst and open a window or quarter vent. If the problem disappears or reduces then it is either an aerodynamic effect or a sympathetic structural one instigated by an aerodynamic effect.

Also check to make sure your hard top seals are present and are capable of doing what they should. Do you have gaps anywhere? - can you slide a piece of thin card under the seal? Are there seals missing or maybe even some rubber bungs missing in the cabin bodywork? And as the worst thought of all, your hardtop isn't bent out of true is it? Another thing comes to mind may be the adjustment of the centre rear catch. Some owners do without them to avoid scratching the bodywork when fitting and removing the top, but perhaps that area requires a little thought.

That's my tuppenceworth - It could take time to sort this one out as (in my limited experience) you seem to have a problem that is unique to your setup, but perhaps others may be along in a short while to put their finger right on it. Lets hope so.

With best wishes,
Duncan, Skye

[mainmansam]

I would agree that you you have a few things going on here. Looking back when mine was up and driving I've had similar issues. First the vibration at 2000 rpm most likely was there always but if your exhaust is touching the underbody or frame anywhere I'd say that the vibration is resinating throught the exhaust at that point. Could be as simple as tune up issues vacuum leak or worse an unbalanced fan blade. Other vibration I'd start by loosening the driveline bolts making sure all is seated properly and tighting back up uniformally. Good luck.

[pcoleman]

hi Don, it seems the problem has appeared when the hardtop has been replaced so I would suspect the screen top locking mechanisms aren't sealing or the rear slam plunger is not maintaining a seal.it does sound like you have 1 window open so if you open both does it go away ?

let us know how you go on

[StagnJag]

It isn't one of the hard top rear windows slightly open is it?

[nib]

With the exhaust you may want to loosen the tailpipe exhaust clamps and all the rest on the right hand side and concentrate on the part that goes through the subframe first, before I finish i tend to give the pipe a sharp shove to see if it will still bounce off the side of the sub frame imitating hitting a pothole.
I do think hardtops were fitted with various weatherstrips along the front edge as an afterthought on the early models,when the breeze outside is concentrated on a small gap it can make some strange noises.
Mark

[Guest]

Thanks folks. Lots to think about there. I agree that I have 2 separate issues and I'll concentrate on the drumming first, as it drives me mad. I'll go into the freezing barn this afternoon and have a good look around the hard top. I do get very loud whistling from the windscreen seal, but I could not feel any air coming in when I slid my hand along it, whilst trying not to go into the ditch at the same time. This is the first time that I have ever fitted it, so maybe something is not quite right. I did notice that the gap between the car and the chrome strip below the rear windows was more on one side than the other, but could not see what I could do about that as both side levers were fully home and the seal appeared to be doing its job. However, I will go over all this lot again. When it stops raining I will go out on the road again and when the MGB comes off the ramps, I'll have a go at the exhaust. I never quite got it back in the same position as it was before I did the diff seal job. The quill shaft bearing that I fitted was the pukkah RHP one, so I'm hoping that it is not that.

Add to this the Austin 7 rebuild (caused by my rutting Stag and slippery right foot) and you will see that I'm fairly busy carwise at the moment!

Daft question for Mainmansam - what exactly do you mean by "driveline bolts"?

[jakesmig]

Regarding your exhaust, loosen all joints and hangers on both banks and give the system a good shake until it hangs in its natural position. Adjust the pipework through the subframe until you are happy with the clearances then tighten up the hanger / clamp at the subframes. Once this is done, tighten up all the remaining clamps / hangers and you should overcome your problem. If you cannot get a natural clearance there may be a fault with the pipework that may need fettling a bit to overcome.
The subframe 'ports' are the only real exhaust restriction, get them right first and the rest should be plain sailing.

Rgds,
john.