Rumbling Transmission

[Balero]

1999 Alero 3400 SFI ~180,000 mi

When I start it up, it idles fine, but when I shift into gear (Fwd OR Rev), I hear this rumbling sound. As soon as it warms up it's OK. Anyone have any ideas?

I'm also currently changing out my LIM gasket, and now taking out the heads since they're really dirty and there's coolant everywhere. Could it be related?

Thanks.

Have you check the fluid level yet?

[Balero]

Fluid is fine, and FYI I changed it out ~80,000.