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Strange noise in 2004 Ford F150

My truck has a rattling noise that is coming from underneath somewhere. It only occurs when you start the engine, or excelerate. When idling there is no noise. When ytou step on the gas and speed up, it sounds as if something is loose. I have taken it to two well known mechanics and they can not find out what it is. The engine runs good, no engine light come on when you are driving. The transmission shifts easy and smooth and they said the drive shaft and ujoints were good. This noise started about three months ago. It has not gotten any worse. I have a new K & N air breather kit on the truck and a new dual exhaust. Two years ago I put a 3 inch lift kit on the truck. The airbreather and new exhaust pipes were put on 3 to 4 months ago. It was put on a lift then raised up off the lift so the tires were not touching and put in neutral. The engine was reved up to about 2500 to 3000 rpm’s - no sound at all. It was put in reverse and backed up at a good rate of speed, no noise. The serpintine belt was taken off and the truck was driven for two blocks. The truck did not heat up but the noise was still there. Everything underneath has been tightened. Could it be air flow? The fly wheel was mentioned as maybe hitting something but it has not gotten any worse. If the fly wheel is hitting somthing, it look like it would have gotten worse. ANY help on this would be appreciated.

Ron H

Was the noise there BEFORE you did the mods?

Check the dual exhaust pipes and hangers. Is anything touching each other (where they shouldn’t) or any other part of the vehicle (eg. frame)?

Those pipes MAY not stay in the (preferred) location when the weight of the vehicle is ON the tires.