Strange rumble on accelleration - need help

[BgShootr]

99 5.4L Triton Eddie Bauer

Recently, I have been getting a strange short rumble when accellerating. It feels like I'm rolling over a patch of road relectors or a patch of rough road. Always happens when applying pedal from a stop or slow crawl and always somewhere before 2500 rpm. RPMs don't jump, trans doesn't shift out of sequence. If I give it hard pedal, it doesn't happen. I don't get this rumble at any other time and it's driving me nuts! No warning lights on the panel and Ex runs fine aside from this. Anyone have an idea what might be the culprit? Recently had the trans drained and re-filled. Starting takes a crank or two extra lately.

 

[green03]

How much throttle are you giving it when it rumbles. Do you mean very light acceleration or medium acceleration? With the windows open do you hear any funny noises coming from the engine?
Matt

 

[BgShootr]

I can't recall ever hearing any noise on the outside. It happens when I'm giving it medium pedal, from a stop or a slow crawl. It sounds awfully familiar to the "rumble strips" posts that I found during some searches on here. I believe it started sometime after I had the trans fluid replaced. If I give it a lot of pedal off the stop, it doesn't seem to do it and I don't get it at highway speed when I'm hitting the pedal to pass someone.

 

[bluebird1]

If you have a 4x4 the cause is most likely from the rear differential clutch packs. I know it sounds like the front, and it is usually worse when you are turning from a stop, but it is the rear! There are several things you can try before going too expensive. 1). change the differential fluid and be certain to add the friction modifier. 2) rotate your tires frequently. The noise in mine improved with the fluid cahnge, but it went away completely when I bought new tires. It seems that all the tires must be the same size or the clutch packs start grabbing as if one wheel was slipping. Hope this helps. Remember the friction modifier - it is essential even if some oils mamufacturers say it is not. The differential requires a full synthetic and that is pricey (5qts) but the modifier is only $5.

 

[BgShootr]

Bluebird, THANKS A TON for the advice!! I had some pretty uneven tires and the right rear was missing the weight and had been given two cans of fix-a-flat over the past six months. I bought the $90 bucks worth of Synthetic Diff Lube and modifier from the stealer on Friday. I checked the stuff Pep Boys put in back in October when they changed my leaking differential gasket and it was thick, brown and nasty - nothing like the clear thinner Syn. I checked my paperwork and saw that they only charged me $57 bucks for the whole diff gasket change. Where was the $90 bucks in lube hidden in that charge????!! Took the Ex back to them this past weekend and told the service manager that they boned it and that I wanted them to do it again, but put the lube and FM that I bought in it, instead of the garbage they pulled off the shelf and put in last time. Did it while I waited without issue and free of charge. Guess what??? Problem is GONE! No slip, no noise, no problem! Thanks for helping me save a ton of money on BS diagnosis and unnecessary work. My Ex is back to its old self. Give yourself a pat on the back and a handshake for me.

-Darren