I'm kind of a newbie on this site,and haven't posted very much,but I could use some help.
I have a strange dull sounding rattle/vibration in my 2005 F150 4wd with the 5.4 engine,and automatic trans.

I can't really explain it but it feels sort of like a dull sounding (not metallic) Rattle or vibration,that seems mostly up front.It kind of feels like I'm driving on a mildly rough road.
Try to imagine the dull heavy sound made by a bunch of hard rubber ***** rattling around in a wooden box.

It happens mostly when I'm at a constant speed like 35-45 mph with no acceleration or deceleration.,just a straight speed.It also seems to increase a tiny bit when slowing down to a stop.
I feel it mostly in the seat,and forward.,only a tiny bit in the steering wheel.

I had the transmission pan dropped and filter changed,the fluid was nice and clean,with no metal particles.and It shifts fine.
I thought it may be the universal joints so when I was at the dealer for an oil change he had a tech look at it and said it needed u-joints.They gave me a price of $600.00,naturally I didn't have them do it.
I took it to a very reputable front end shop who told me the u-joints were fine,he dropped the drive shaft to inspect them and found them to be ok.

I jacked it up spun the front wheels thinking it may have been a bad hub,but no noise,all seemed fine.The engine idles fine with no vibration,and is fine at higher rpms as well

The truck has 87,000 on it,and this problem started just a few days ago.
The front end shop was at a loss for what it could be.I hear everything from broken fins in the torque converter to a bad harmonic balancer.on the motor.

It's driving me nuts,and I have to make a 1,000.00 mile trip,but this worries me.
Any thoughts,or insight on what it could be would be very much appreciated.
Thanks guys

 

Does it do this after it shifts into od?, or will it do it with od off also?.

 

It does it no matter if its in or out on the overdrive. Funny thing is that when I just started the engine,a few minutes ago, it was running rough, like it was missfiring, and I had it in park. I increased the rpms a bit and it shook more. This was the first time I felt any difference in the engine.So, once I started driving, it was doing the same as it did before with the vibration? When I stopped at a light the engine seemed to idle fine..
I stopped the truck, put it in park, and brought the rpms up to1500 and tried keeping it there, but I noticed that the rpms would drop very slowly, and then increase very slowly back to 1500
I'm wondering if its a missfire in the engine that gets worse under a load.
Im at a loss with this,it bringing into a shop to have them look at it on monday