LS1 4L60e loud noise while shifting at WoT

[JBAB64F]

Sorry, accidentally posted on 5th gen site first.

LS1 4L60e loud noise while shifting at WoT
Hello all. New to LS1.com. Just finished up doing my first LS swap into a 64 Fairlane. Hope that doesn't anger too many here......
If anyone is still reading after the previous sacrilege, I have an issue I was hoping to get some guidance on.
The motor / transmission runs great during normal driving. But, during WoT shifting, I get a loud noise right when the transmission shifts. Happens in any gear.
Wiring harness and computer are new (PSI conversion), new TPS, IAT, new plug wires and plugs. headers and cold air intake. No other mods yet. No codes are thrown.
If I let off the gas right before a shift at WoT, there is no noise, everything sounds normal.
Engine/tranny combo is out of a 99 trans-am with only 67k miles.
I would hate to pull the transmission for rebuild if it's not the problem.

I can send/post video of sound, just haven't figured out how to do that yet.

Thanks!

[mrr23]

transmission mount torn?

[JBAB64F]

It's new but I can check again

[JBAB64F]

transmission mounts good.

Here is a link to the video. Maybe that will explain it better.

[mrr23]

somethings moving

[0rion]

I agree.....something is moving on the shift. Hard to tell where it's coming from front/rear. What set up do you have under the car? I've heard torque arms pop like that if they weren't torqued down well but like I say....can't tell where it's coming from.

[JBAB64F]

The LS swap was just part of the project. I removed the shock towers and installed a mustang II suspension. I have a 3-link out back, but that's been there for 6 years. I will check the torque arms again since everything is new. Thanks