5.0 exhaust makes rattle noise at start up

[bav0004]

I have a 1996 Ex 5.0 with 300K+ miles on it. I just got a new Magnaflow Muffler put on, and afterwards the exhaust guy said i had an exhaust leak. Now when ever i start it up first thing in the morning, it makes a ratteling sound coming from around the Passenger side Wheel area, but when ive looked, i cant seem to find any leak. The noise eventually goes away after 20-30 seconds. Any one have ideas as to what this could be?? I have no check engine lights on, and the car still runs like a champ, just worried about that first start up rattle noise.

 

[Beerking27]

could be the heatshield around the catalytic converters, my old explorer did that and made a hell of a racket all of the time

 

[bav0004]

ok, but if it was the heat shield wouldnt it make the noise all the time while i drive it?? this rattle goes away after less then a minute of the car warming up.

 

[Beerking27]

Well you said you didnt have this noise until the exhaust guy put on your muffler? He could have always messed something up.

Also an exhaust leak sounds more like a whisle and you would hear it when stepping down on the throttle, rattling sounds more like the cats to me. mine still sounds like theres popcorn going off inside of them when i turn the car off

 

[bav0004]

Ok, ill take a look at the heat shields then later today see if they came loose.

 

[bav0004]

Finally got to look up under the truck today. Been Raining the past few days, and i didnt see any of the heat shields that were loose. But the exhaust does seem to make that popcorn popping sound when i turn it off after its warmed up, that you were describing. Any ideas of anything else it could be??