Banging from tranmission tunnel

I recently had my tranny mount replaced in the car. It was banging pretty loud when shifting under heavy acceleration. This mount has broke twice in 10 yrs. It's a fairly high hp car (modded), so I didnt really think anything of it. My problem is I just had the mount replaced again, about 900 miles ago, and the car is doing the same thing as before. I had it done at NTB (which was a mistake in the first place) and the warranty it for 3 months/3000 miles. So I took it back today to have it checked, and they are telling me it is my exhaust that is banging when the car shifts now, and the trans mount is fine. The banging is coming from the trans tunnel area, trans fluid etc is fine, nothing wrong with the trans. My exhaust does not vibrate or make any noise when the car is running. When I asked if they could fix the exhaust mounts, they declined to do the work and said to take it to a muffler shop. So I guess my question is, could this actually be an unrelated exhaust issue or are they just trying to get out of fixing a problem they caused? (I.E. Defective/wrong trans mount) Any info would be great. Thanks.