A noise from underneath

[sam79]

Sorry to be off topic again.

I'm still helping my brother in finding a decent Chevy Van and we have found a real nice one today. There's just one thing that is a bit of a worry for us.

When we took it for a test-drive we heart a banging noise from underneath when taking a hard right turn. The owner thought it was the mounting-rubber that was at the end of its life and therefore the transmission was banging against the support. Could this be the problem or should I think of something else that causes the banging noise, such as a bad tranny.
The van was equipped with a 305 engine with a TH400 transmission. The thing I thought about is that this Van came with a TH350 and they have changed it to a TH400 without swapping the drive-axle. So I thought the drive-axle could cause the banging noise because it's not the correct length.
I'm a completely ridiculous thinking this could be the problem. If so, please say so!

Any help in this matter would be very appreciated by me en even more by my young brother.