I have an '86 automatic coupe. There is a light tapping sound coming from underneath the car... seems to be from about where the catalytic converter is. It is definitely RPM related - not vehicle speed related. At 1500 RPM, it's most noticeable regardless of what gear the tranny is in. (It sound like hold a card in the spokes of a spinning bicycle wheel). You can hear it from inside the car, but not outside. It seems to be worse when the car is cold. There is no vibration associated with this sound.
My first reaction was a cat going bad. The previous owner said that the cat is hollowed out. Although I'll be going under the car this weekend to investigate, does anybody on the list have any ideas of what it might be?
Thanks,
Ed

 

Here's a wild guess: could it be a very small exhaust leak? The card in the bicycle spoke example you gave made that jump right to my mind.....

 

I agree, sounds like exhaust leak @ the manifold to cylinder head. That is exactly the characteristics it exhibits because cold engine manifold is shrunk, heated it expands. [8D]

 

Well, I got under the car and listened for the source of the tapping. Near as I can tell, it's coming from the fuel lines that run under the rocker on the right side of the car. I touched the lines, and one of them was throbbing in association with the tapping sound.
Any idea why a fuel line would do this? It is very noticeable when inside the car with the doors and windows closed. I realize fuel pumps make a little sound, but this is loud enough to be heard over the grumble of the engine and hollowed out cat/loud exhaust.
Thanks,
Ed

 

Are the fuel lines secure? All the clamps present? If they are, you need to install fuel pressure gauge and see if pressure is pulsating. Should be consistant pressure. If not, probably time to replace fuel pump. [8D]

 

The A.I.R tube from the engine to the Catalytic converter might be vibrating.

 

Thanks for the headsup - what brand of clutch & pressure plate would you recommend and did you go back with a new flywheel?
Thanks

 

Since it now sounds like it's coming from the fuel lines, I'll borrow a fuel pressure tester this weekend and check out the fuel pressure.
In the meantime, I did some internet research on TSBs for the 'vette.
Does anybody know anything about TSB #86234, issued April 87? It apparently addresses a Fuel Return Pipe Rattle/Knock - almost sounds like what ails my car...

Ed

 

I looked through TSB's on my Direct Hit website and nothing yet. I will try and look through Alldata @ my shop when I have time and see if any are listed in there.

 

Found this: