RCooper

Hello everyone longtime no see (job blocks all MSG board sites). I recently had my mechanic replace my muffler yesterday. Later in day I noticed that I heard rattling coming from muffler whenever I held my brakes. I also heard brief knocking whenever I hit a bump and this was coming from the muffler also. I took the car back in and he fixed the rattling during braking said it was due to the muffler being close to car body. However I still hear the slight bumping noise when I hit a bump. Any ideas on what tell him to do to stop this issue? Don't want to keep going into the shop for this.