revs dip sharply for a split second

Hi,

I have noticed on a few occasions that the tachometer will abruptly dip for an instant and then return to normal. I believe I felt a slight shudder and heard a noise from the engine at the same time but this happens so rapidly I am unsure. This happened a few times with a cold/cool engine at low revs < 30mph, and once at highway speed.

Any ideas, or other information I can provide?

Dave