Strange noise on downshift

Was coming home today with trailer in tow (5000 lb), had already driven for about 1 hour, hit a hill and when tranny shifted down, heard a strange noise. I wasn't sure what it was, thought that i might have hit the rumble strip for a second, it did it again several miles later, just on a downshift, on a slight hill. I couldn't quite place it, not really sure if it was trans or something else.
Went another 20 miles or so and it did it again, had a slight shudder to it this time, in the trans for sure, tried to narrow it down to OD to Direct or if it was the torque lock up clutch, but the darn thing never missed a beat for the rest of the trip.
We just started pulling the trailer, this is only the 3rd trip, but never had any problems before, had to climb a couple of 13% hills, never missed a beat there, was just coming out of OD or lock up.
Any ideas???
Thanks
Brian

Could it be the exhaust pipe hitting the frame?

I'll take a look and see, truck is up on blocks right now, blew a wheel seal on the left rear, waiting for parts.
Thanks for the reply
Brian

Nope, not the exhaust. Will see if it does it without the trailer behind.

That is some kind of solenoid and I have the same thing happening. I bet someone will chime in on what it is. I heard it on some radio fix it channel.

Really???
If someone knows, your $0.02 would be appreciated.
I was going to swap over to TranSyn but am going to wait now in case it is the trans. Don't want to waste that expensive oil. ;)