I was coming to a stop in my 98 Ford Explorer, after the stop i pressed the gas and heard aloud thump rite before my truck start coasting, when I put it in park, the gears sound like they where grinding, and my explorer was still rolling, what might be the problem?

indicates faulty brake booster

16 costly car noises - Bankrate.com

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BankrateYou should not ignore certain thumping, banging, clicking and squealing sounds. These sounds often rear their heads or intensify whenever the driver takes ... Hissing or sizzling under the hood: Heard when the engine is first shut off, .... my rear right wheel makes a loud bang when switching from park to drive/reverse.

Sounds coming from drivers side front wheel - AnandTech Forums

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Dec 1, 2009 - 21 posts - ‎10 authors
Sounds coming from drivers side front wheel The Garage. ... slightly left, I get a slight metalish-squealing sound coming from the drivers side front wheel area. ... but before I start the wild and expensive goose chase of bringing it to mechanics I ..... Those twoloud clunks when turning hard right give it away.

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CarGurusDec 22, 2009 - Thumping from front end - I have a hard thumping noise coming from ... without problem then I'll take a turn or something and it will start. ... I just recentlystarted having a rubbing and thumping and squeaking noise in the right front sidewhen driving ... Thumping sound coming from under the truck 4 answers.

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Take it back to whatever shop installed the rebuilt engine and make them troubleshoot the problem. There are no coincidences ...
However, "loud thumping noises" don't sound good at all.

I'm not sure what you mean by "Thumping". Please explain if this thumping is engine or body related. Try to describe the symptoms with more detail. Thumping to me, could refer to a Random Misfire, Internal transmission failure, Using the 4WD while on the wrong road surface or the tires are not all the same size, a defective U-joint (in the drive-shaft), a defect inside the rear drive-axle (ring&pinion), maybe a defect in a tire (not balanced or a bubble in the side-wall or tread), wheel bearing, ball-joint, tie-rod, or a needed alignment (observe if the vehicle pulls to one side and look for irregular tire wear). Let me know, good luck.

Check your belts, it might be time to replace them.

I suspect there will be something loose in a CV joint, under load it will be held in place but when it is not under load it flaps around causing the thumping. You should be able to identify which side of the vehicle it is coming from then a mechanic should see it ASAP because if it lets go completely it will not only be dangerous and stop the vehicle from going anywhere it will be very expensive to fix the colateral damage.

it sounds like you have a universal joint that has gone bad

if your lucky, its just a u-joint going out.

try replacing the u-joint which connects the driveshaft to the rear end(differrential), and if that does not solve it, then have your differrential checked out (a bad differential usually wont get all the power to the wheels that its supposed to) or just have it replaced, possibly with a junked differrential form a wrecked vehicle with low miles.

If it is "right after you select 4x4" then it sound normal if you have miles on it. If the thump is on one side or the other and when braking then a strut bushing

sounds more like a ujoint or a center support for the drive shaft. also check and make sure that the flywheel bolts have not come loose. also check the harmonic balancer and make sure it is not seperating