Hi,
Chances are something needs adjusted. When the engine is starting, it likely kicks a bit which can cause the exhaust to move. If the system was reinstalled too close to a component under the vehicle, it can hit.
Have a helper start the engine while you (safely) watch under the vehicle to identify where the noise is coming from. Watch near the muffler to see if you notice it moving.
Also, when the vehicle is cold, grab the exhaust at different points and shake it to see if you can recreate the sound.
Let me know if this helps. Also, if I installed it and there was a noise, I would fix it. I don't know what kind of relationship you have with the shop, but just because it's an aftermarket muffler doesn't mean it should hit under the vehicle. They really should readjust it to prevent it.
Let me know if you have other questions.
Take care,
Joe
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Saturday, September 19th, 2020 AT 9:23 PM