I got my flex pipe replaced the other day and not there is a buzzing sound coming from the flex pipe area. I noticed when they cut off the flex pipe there was a pipe inside a pipe right where the O2 sensor is (my car is obd 1 92') and I was thinking that maybe that inside pipe is making the buzzing sound? I took it back and they checked the cat heat shields and the exhaust heat shiels and they were all tight and had good clearences. It sound horrible right now and I need to get the damn thing fixed. anyone got pics of this pipe inside pipe exhaust header design??

all the header heat shiels and supports are fine too and so are the intake bolts.

when they replaced the flex pipe they welded on a short section of tubing ahead of the flex pipe (between the o2 sensor and the begining of the flex pipe) if that matters.

 

Take off the down pipe and look inside to see if one of the inside pipes has broken loose and is rattling inside. also check for any dents on the outside pipe as this can reduce clearance on the inside and cause it to rattle as well. There are two possible fixes. On mine it was broken close enough to the opening that I could get in there with a welder and weld the pipe back together. Some others have cut the outside pipe bent it in to touch the inside pipe then weld it all together.

 

could it be that the downpipe is hitting the motormount i think that was what was happening to mine and they had to cut a piece of it off