Rumble when stopping or backign up

[ridiculously_necessary]

When I stop more quickly than usual the truck starts to knock, or a better description would be a rumble. When I back up its a very heavy rumble, like the motor is shifting and rubbing on something. The mounts appear to be fine. It does not do this in drive. Someone mentioned U joints to me, what do you guys think?

[ono_55]

have you tryed power braking it with someone watching the motor?

[ridiculously_necessary]

nope. but i powerbraked a while ago just messing around and dont recall hearing the noise, ill do that today again to be sure.

[brennan]

My truck did that when I first put the 3" downpipe on, it only came in contact with the firewall during braking or in reverse. Thats only possible if you have aftermarket exhaust though.