Last compression test was over 70K miles ago (6yrs) but at the time they were within 4% of each other (202, 201 194, 200). I do it when changing the plugs but last time the plugs were changed at the dealership for free as part of a EPA required recall. So it's been a while but the plugs aren't due just yet.

It doesn't happen at idle, and usually not in 1st or 2nd gear at all. Typically once I make the shift into 3rd it starts coming on and I hear it most in 3rd 4th and 5th and only under load. Its worse with more throttle. This makes it difficult to diagnose since I havent found a way to listen to the exhaust with the car in 3rd gear under load. If I were a rich man I'd have a Dyno in the garage but that day hasn't come yet.

It sounds like popping more than metal-on metal. Not to say that it isn't really muffled metal on metal but it isn't a sharp, high pitched sound.

Are exhaust leaks typically rythmic sounding or irregular. For example, I had a dying CV joint and that gave a very regular, periodic tap. This one literally sounds like popcorn, a lot in rapid succession, then a split-second pause, then it picks up. There's a randomness to it.

I've heard of plugging the exhuast pipe partially to make the small leaks whistle. Does this really work?

 

nevermind about the rod bearing then

 

In addition to the exhaust components I already mentioned, you should also consider a loose or cracked exhaust heat shield.

 

itinerant: It does sound a bit like that but I can't get it to do it in the garage, I have to be giving it some power when cold in a higher gear.