knocking sound after warmup

A knocking sound presented itself about a week ago. From inside the cab, it sounds like the handle of a screwdriver being smacked against an aluminum housing that's full of oil. It's neither a hollow sound nor does it appear to be a solid sound. Here are the interesting variables:

(1) The sound is absent when everything's cold.
(2) The sound is absent if I let the engine warm up to operating temp but do not drive it anywhere.
(3) The sound is present after driving about five miles.
(4) The sound runs concurrent with engine RPM

So, I'm thinking tranny. It's a 4L80E.

Last night I crawled underneath with my screwdriver/stethoscope and auscultated various locations.

(1) The sound is almost absent in the engine oil pan.
(2) The sound is very prominent around the TC housing cover.
(3) There is also a noticeable "clicking" noise inside the tranny pan.

The transmission does not exhibit any evidence of slippage. And, there are no codes when jumping pins A&B on the ALDL.

Any ideas?

I assume that your oil pressure still looks good? I would go ahead and change the oil or at least remove the current oil filter and cut it open to look for any surprises. If it is a rod bearing causing the knock, you will most likely see some debris in the filter. I hope it is something in the transmission. Keep us posted.

Engine oil pressure is 60 @ startup and drops to 50 @ operating temp. She's due for an oil change this weekend - will inspect filter.

However, given the variables listed in my first post, I have my doubts about anything engine-related. I just don't know anything about automatic trannys to know what would cause this sound that presents only after the thing is warmed up after driving a bit.

I was hoping somebody w/ trans experience could shed some light on the inner workings of the beast.