Currently at somewhat of a loss on my 2001 DX right now....

So, the car has slowly developed a sound at startup. I attempted to use a hose to isolate the location, and it's somewhere in the vicinity of the top right of the exhaust manifold. At start (and idle), the sound is almost a wah or wub sound. Upon acceleration, it almost sounds helicopter.

So, I'm driving on the interstate on Sunday, and the car throws a CEL, and seems to lose power. I immediately pulled over, and towed it home.

Admittedly, I made a mistake -- I pulled the battery, and reset the ECU. I wasn't going to have a scanner until the next day, and was curious if car would throw the code again...and, here's the problem...it hasn't.

Ran the car for 20 minutes on Monday, no code. Drove the car to work today (120 miles), no code. The noise is still present, however, and at times -- it sounds awful. But, the idle is good, acceleration is fine, etc...if you were deaf, you'd think it was fine.

Any ideas?