Muffler making rattling noise

[Last Lemming]

So I added an AA catted DP a couple of weeks ago and tonight I went stage 2. Just came back from a first test drive after the tune and when I parked the car while still running I heard what appears sound like small bits of something rattling in muffler. Could this be the cells coming apart? Any other ideas? I didn’t push the car too hard btw. Maybe 80%.

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[Davidr3]

Check the valve on the muffler

 

[Last Lemming]

Its not unfortunately. With the car off I just went out and hit the bottom of the muffler with a mallet and I could here the bits bouncing around

 

[armenh7]

That's the sound of metal contracting after being heated and is now cooling down. Completely normal.

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