Strange Exhaust Noise

Dear Chumps,

I have a really exhausting problem…

My car-nuts girlfriend and I co-own a 1941 Chevy 1/2ton stakebed which emits an annoying ‘chirping’ or ‘squeeking’ sound from the left-hand exhaust. Our truck has a mild 350 chevy crate motor with cast iron exhaust manifolds and dual exhaust without any crossover pipes.

Using an engine stethoscope we were able to localize the point where the noise was loudest, which initially appeared to be at the exit point of the left muffler. Ah…hah! I thought! Its muffler resonance, so we replaced the mufler…to no avail! The "chirping, though somewhat reduced, continued. At this point I should mention that there are absolutely no exhaust leaks or squeaky muffler hangars as my exhaust specialist (who we are slowly making rich) would attest! So finally in desperation we replaced the entire left-hand exhaust system (again) starting from the manifold back, using a top-quality glass-packed muffler to suppress resonsnce. And eureka! The exhasut sounds terriffic…except for that pesky ‘chirping’ noise!!!

I should probably point out that the “chirps” are synchronized with engine RPM, and the volume is most pronounced at low RPM, especially with the transmission in ‘drive’. It becomes increasigly quiet as RPM is increased to a point where it becomes inaudible at cruising speed. About a half-dozen folks have been all over and under the truck with stethoscopes without discovering any bad alternator or water pump bearings or air leaks or anything elso that would help to diagnose the problem. My (now wealthy) exhaust mechanic says he’s never heard anything like it! I should also mention that the noise is extremely pronounced upon cold-starting the engine, at which time its louder that the exhaust note!!!

Could this noise be generated inside the engine itself, or perhaps originate in the exhaust manifold? Someone suggested that it could be an induction leak, but we should be able to rule that out by listening under the hood, not under the chassis! Please help! We sure would like our ‘baby’ to sound like the hot rod it is rather that the old junker it sounds like at every stop light!

I. M Exhausted