Intermitant Knock

[Stagless Steve]

I went to look at a Stag this weekend. It was a 1972 model with the original Triumph V8. The owner seemed to be quite honest and open and pointed out an occasional knock from the engine on tickover. The revs would dip a little at the same time. The tickover did not seem to be unusually low. Is this something to be scared of, or is it normal for a Stag V8 to give this occasional knock? Grateful for any advice.
Steve

[V Mad]

No that's not normal. It could be a sticking valve. A vacuum gauge test might tell something.

[Stagless Steve]

I feared as much. Thanks for that.