Hard acceleration issue

[Coach]

Thought I would try again and post this a 2nd time to see if anyone has any thoughts. Upon hard acceleration from a complete stop or if almost at a complete stop and then hard acceleration, I am getting a loud popping sound coming from right under the floor, right where the floor shifter is. It is an intermittent problem and has happened only 3-4 times in the last month. Once it happens, I let off the gas immediately and it goes away and I can continue driving with no issues. Not sure if this could be a transmission mount, u-joint, driveshaft, or an actual transmission problem (700r4 - nothing but problems with that tranny, eventually I will be going to a TH350). Thanks for any help/advice.