I have a 2010 f150 5.4 with 140,000kms on it. Lately I hear this awful rattling noise when towing, but only going uphill or small hills. On straight roads I hear nothing, but towing uphill it’s unbelievable. It’s like something trying to engage but ain’t. And as soon as I let off gas it goes away instantly. So I can duplicate it under engine load. And now it’s starting to do it not towing, just simply any hill. Vacuum issue?

The engine sounds smooth, no leaking or no burning oil. Totally maintained this truck. Aside from towing I’ve babied it. It’s not The take manifold or exhaust as I’ve checked that.

could this be torque converter? Transfer case? Transmission bearings?? Or from what other threads have mentioned in similar issues could it be the IWE valve or solenoid?

any thoughts?

It was the actuator check valve. $18 part. Took 30 seconds to replace, no tools required.
Mechanic replaced both IWE actuators and wheels bearings twice. Still grinded. That’s when I replaced on my own the check valve. Instantly went away...