1999 challanger 4x4 has a harmonic rumbling noise that comes in at 75-85klm/hr this causes a droning sound in the interior??

possibility of CV joint but then you should get the same sound at low speeds also.

if you getting this sound only at 2000-2500 rpm may be over advance ignition timing

a noise from a diff are , normally a high frequency harmonic sound while driving is from diff pinion bearings
if you decelerate and you get a noise normally as a low grumble ( low drumm noise )to a frequency noise is again the diff pinion bearings
you can distinguish the noise from power on to deceleration
the noise can be most noticeable with a slight load ( 2 people travelling at a steady speed say 60- 80 klm/hr) as a steady but harmonic pitch variation
the noise varies as the load is transferred from the bearing immediately behind the pinion gear to the bearing immediately behind the drive flange or in your case driven gear from the gear box
it is caused by a slight movement of the pinion which varies the correct mesh with the crown wheel , resulting in the whining sound
sorry but noise sounds are very hard to describe

The transmission was rebuilt-- but not the differential
within the same case

You hear worn out bearing noise

Spin that wheel by hand as fast as you can. If you can hear the noise coming from right behind the tire it's probably a wheel bearing.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

Wheel bearing is likely cause. Listen to see if it's coming from a wheel and which one. Jack up the wheel, take off wheel and see if you can feel play in the bearing. may be hard to detect.

I believe the bearings in the front wheel are about to come to the end.
Have it checked in a garage.... NOW! otherwise you will be driving on 3 wheels... sooon.
Iceman.

hi ruby ?? now we have a good idea as to what is the problem thanks to your description on noise ! this sounds very much from what you describe that it is front wheel bearing on its way out /worne the bearing fits inside the hub assy either offside front or nearside front the actual driveshafts run inside these bearings cv joints as they are known not uncommon fault on many front wheel drive cars however it depends on which side you suspect the noise/droning is coming from?? you can try listening with a passenger to pin point ? or you could jack each front wheel in turn and rotate wheel also try rocking/tilt wheel top and bottom to feel for any undue movement ? there should be none if ok this is not an expensive job to repair howeveri would suggest you shop round motor supermarts or shops for price of new bearing 1st not dealerships! if you have haynes manual it will describe the proceedure for doing this job normally 1 hr to change a little longer if you have a friend ?? who could do it for you saving££££s in garage costs ?labour charges are costly try this ? whilst driving on a quiet road if you move steering right and left you may find droning sound more on one side? or may disappear either on right steer or left? this may help in verifying rogue/worne bearing to you ?? for your info i know that cost of one of thee bearings on a cleo 94 year cost £18 very similar to size on yours ? hope this helps ? ps whilstyou are checking wheels as above ? yu might as whel check the b/pads wear/thickness just in case you may need to replace them too whilst doing bearings for peace of mind for future and mot ? not aware of your location so as to offer places/stores/motor factors to try /price up parts ? obviously trade is less so wish you luck and hope all above is lear enough for you to understand come back if you req more info/assistance ?

I have this issue with my '05 Accord hybrid and just lived with it for years. It seems the problem of the low hum or rumble is common for many people with the Accord in recent years. It appears that this can be caused by more than one possible issue. Besides the obvious causes that can occur with any car and any model like tire/alignment, bad suspension or other such things, the most common cause for the droning sound we have here appears to be a malfunction with the electronic Active Noice Cancellation (ANC) unit. The same or very similar sound can be caused also by a bad motor mount. It's uncertain how to tell which one in any given case. I'm pretty sure that if it happens only in the summer months when it's warmer outside, it is more likely the motor mount and not the electronics. I also have the feeling that if you can tell it sounds like it's coming from your front speakers and not the engine compartment then it is probably the ANC. Honda has recognized this particular issue but there was no recall since statistically it happens with very few cars. A kit was created for the repair. Part 06390-SRD-307 Speaker Kit - comes with the ANC unit and replacement speakers. This can cost close to $800 USD at the dealer unless your car is still under warranty. It's also possible to buy the kit outright for much less and install it yourself with instruction. Good luck.

low profile tires can cause this droneing noise , or if the wheels have to much air in them , id put 32 in the tyres and try it , if this doesnt work it could be a wheel bearing

guess i should make this so you can rate the solution ... thanks