I have a 1993 325 i 148K miles… Runs great with many after market performance enhancements. After the car warms up I hear a sound under the hood… rear of the engine or below that sounds like a combination of a sewing machine and a popcorn popper. Weird… and is somewhat loud. The car can cool down, then start it up and it doesn’t make the sound until it has been running a little while.
No sensor light on… I have not replaced the Catalytic converter … I have owned the car since it had 43K miles on it.
Signed Perplexed…