I drive a 1991 fc5s. I've got a straight piped exhaust with a section of flex pipe a racing bear header. When I turn the car on Its fine. Once I start driving and warm it up I hear a weird rattling sound coming from the rear of the car. I can hear it inside the car and I can hear it when I get out. I warm it up by driving for a while under 2k RPMs. Kinda sounds like a belt buckle in a drier. There's nothing in the muffler. It runs fine but the sound is concerning.

 

what can you do to influence the sound? Does is increase or decrease with RPM's or Speed etc?

 

There are numerous heat shields around the gas tank and old stock exhaust area that you should check to see if they are loose.
Also there is a baffle in the gas tank that can due to age,etc,break off and flop around in the tank.It surrounds the pump sender assembly...

 

http://youtu.be/MJya-KiTHZ4

 

Would that be a repair job or a replace job?

 

..weird sound..likely Possessed..lol.(sorry man,had to say that.!)

See if you can get the pump assembly out of the tank.Check the baffle in the tank.
My guess is that the baffle in the tank is being "clanged around" by the suction of the fuel pump.

I know it's only a guess,but hell nobody can pinpoint it..so far..!

 

Found the problem. One of the hanger in the back of the exhaust came loose. The sound was coming from the metal piece in the hanger hitting the heat shield next to the gas tank.