2003 Explorer feels like driving over expansion joints on deceleration

[Schlotzky]

When cruising or accelerating, the truck drives just like it should with a 4L and 80,000 miles, and letting off the gas at anything under 40 feels like anything else. The problem is when you let off the gas above 40, it feels like I'm driving over expansion joints in the road even on new asphalt. From 60, without using the brakes (or with, doesn't really matter) the truck will shake and sound just like I'm driving over a somewhat minor expansion joints, other times seemingly larger until about 35mph where it will smooth out. The interval between these jolts is consistent at any speed, and I would say they are about 3/4 second apart.

In mind the timing rules out any tire or drive line issue, but I'm stumped. Unless 4wd is trying to engage itself or something? Or a brake malfunction? No lights on the dash, although I did just replace the rear speed sensor to cure the ABS light.

Ideas?

[Schlotzky]

Bump..

[shorod]

If you step on the throttle as soon as the truck starts making the noise what happens? Also, have you tried carefully shifting to neutral when decelerating to see if that has an impact. If the noise is still present in neutral, then what if you gently increase the engine speed while still in neutral?

-Rod

[Schlotzky]

If I step on the throttle as soon as it starts doing it, it instantly goes away. Only decelerating. I tried shifting into neutral, and while it seemed to be less noticeable, it still seemed present. I'll have to try giving a bit of gas in neutral to see if that has an effect.

[Schlotzky]

I drove it a bit more and decided it goes away when I shift into neutral. I think what I was feeling were imperfections in the road. So it feels like I'm driving over the expansion joints only in drive, only when decelerating over 30 or so mph, and the timing is completely independent of the speed I'm going otherwise.

[skeeter123]

Possibly an engine mount is busted. What does it do in reverse? Try accelerating in reverse (safely) see if it does it. Also, you can put on the brakes, put it into drive, and put on some gas (helps to have friend) and see if the engine raises up off a broken engine mount. Try this in reverse as well.

[wmax]

Did you locate the source of the noise/thump? I have the same exact issue in my Explorer.