All Trucks, Campers, Trailers, RV's and Motor Homes - Sound unique to Ford?

My 2005 F-150 4x4 just started making a strange sound only when under power. It's hard to describe but it sounds a bit like a rattle snake & it seems to be coming from the drive line somewhere. It only does it when I am accelerating & especially when I pull my trailer. It isn't in 4 WD so it can't be the transfer case, u-joints are tight, it's not a whine like a bearing or a growl like a rear end. If I put my foot on the brake & bring rpm's up to 3,000 there is no sound but as soon as I let off the brake & start moving, there it is. I'm at a loss & so is a buddy who is a GM mechanic. Any thoughts?

is it an automatic? HOw big trailer you towing? I have friend with F350 made similar noise, wasnt rear, ujoints as you state as well. He replaced all those parts. It ended up being is torque converter whinning. He was running really hot and didnt know it since he has no trans gauge. Iam not sure what happened to the torque converter but he narrowed it down to that with a trans guy.

a rattle snake?

like in the rattle or the hiss?

I would look to either heat shields on the exhaust system or possible the core of the catalytic converter itself.

The trailer is 4,500lbs & in 6 weeks I'm upgrading to one that is approx 7,000lbs. Auto trans. Like in the rattle. Definitely not heat shields, everything is tight there & as for the catalytic converter that should still rattle at 3,000 rpm's with foot on the brake. Sure hope it isn't tranny or torque converter. I'm thinking expensive.

Okay, when I put it into 4wd the noise stops. I'm thinking tranny for sure.

putting it into 4wd should not affect anything in the trrans itself but it will affect the drive chain in the transfer case if there is a chain.

if a chain, what is happening would make sense. in 2 wd, there would be no tension on the chain so it would tend to flop around freely on both sides of the gap between the two sprockets. If in 4 wd, there would be tension on the chain so it would be more taught on one side and if that side is slapping the transfer case, there would be your noise/ no noise.

that would also explain the no noise without moving situation.

nap, that's a perfect answer. When the sound first started I compared it to a chain running loose on a sprocket. Makes sense. Thanks.

the noise is caused by the 4x4 auto hub trying to engage when it is 'not' in 4x4.
several causes.there is a revised solonoid behind the battery
(new one has a 'rain hat'-to keep water out)
also pull vacumm hoses off at auto hub-behind wheel,if hoses have water in them the hub should be replaced.
while you have one side hose off, start the eng-test for vacumm at the hose.
a good place to look for a vac leak is-remove battery & battery tray-inspect the lines going to the solonoid for holes,or deteriorated.

the noise is caused by the 4x4 auto hub trying to engage when it is 'not' in 4x4.
several causes.there is a revised solonoid behind the battery
(new one has a 'rain hat'-to keep water out)
also pull vacumm hoses off at auto hub-behind wheel,if hoses have water in them the hub should be replaced.
while you have one side hose off, start the eng-test for vacumm at the hose.
a good place to look for a vac leak is-remove battery & battery tray-inspect the lines going to the solonoid for holes,or deteriorated.

After I had it fixed, I read this & you're right on. Replaced the left side hub & installed new solenoid & all is good. Thanks all.