Gear question on Marauder "cousin" FN145

[Matt In Detroit]

Im hoping I can get a good answer on my Town Car from you guys. I think youre a more technical buch than the Town Car forum guys. Ive got a 2003 TC, as you know it shares the same rear end. Im trying to track down a vibration issue and im wondering if it can be the rear end gear. Going down the road under 50 mph the car is smooth as hell and quiet as you might expect from a TC. From 55 to about 70 there is a distinct vibration in the front floor that feels like a badly out of balance wheel or bent rim to me. I describe it as a loud hum. The tires are new and have been balanced, the brakes have been done, both front hubs feel fine and smooth with no play. Car drives straight and does not pull. So about the rear, if I jack the car up and spin the wheel you hear a distinct repetitive gear sound like as if the pinion and ring arent meshing smoothly. My experience is the rear should be silent and smooth sounding, the sound speeds up as you spin the tire faster. Knock your knuckles on a desk, that is about how it sounds. So im wondering if the rear end is making this vibration, transferring it up the driveshaft to the motor and trans and crossmember making it feel like its from the front end. The rear outer bearings and shafts in the rear were just redone so thats not it.

Any thoughts? Id love to hear them. TIA

[RF Overlord]

Pull the diff cover and see if you have any missing teeth.

One more thing...you said the tires were new and have been balanced. Were they Road Force balanced? I had a brand new tire on the front of the Marauder that would balance just fine on a dynamic machine, but still had a vibration. Road force balancing revealed that it had a hard spot. The local BFG dealer replaced it immediately with no argument.

[larryo340]

What mileage is on car? I have only seen gear whine when the ring and pinion wear badly, which gets louder with speed and won't drop off as speed increases. Have you checked the U-joints for wear, and checked the driveshaft to see if you lost a balancing wieght?

[Matt In Detroit]

Car has 70k on it. I plan to take the car back to the tire shop since the tires came with lifetime balancing, I will ask about the road force balancing. The U joints are new, vibe was there before and after replacing them. I will take a good look at the shaft and see if it looks like it lost a weight.

[Marauderjack]

Sounds like the drive shaft to me.....unbolt it at the pinion flange and turn it 180* and bolt it back on......if it is better (or worse) you should have the DS rebalanced or get a high performance one!!:beer:

Good Luck!!

Marauderjack:burnout:

[Stranger in the Black Sedan]

Certainly could be gears, if the pinion bearing preload has backed off from the awesome crush collars they used in the stock rears. Anything is possible. Checking pinion bearing preload and backlash should tell you something.