YuCrew

I have a 1999 ML 430 with 133, 000 miles. Early this morning I was turning relatively slowly into a parking when I heard a bang...thought I ran over something. Got out (too early for any traffic around) looked around and under and saw nothing. As I rolled forward to park, I heard what sounded like gears clicking/grinding - clicks/grinds faster as speed increase. It was so loud the few people in the parking lot turned to see who was making all that noise! I feared the worst, presuming drive shaft, differential, transfer case or transmission.

My mind immediately raced through the work I've done over the last 2 weeks. Oil change...that can't be it unless it's all drained out because I forgot the plug. But that was 100 miles ago. One new O2 sensor. That's not it unless I left a tool. Drive shaft center support bearing replaced 300 miles ago along with transfer case fluid (Castrol Dexron III) change. No leaks at either transfer case fill or drain plug.

What could it be? I didn't want to drive it any further fearing a noise that loud is an indicator of major $$ problem. Put car in park, turned off the engine. Looked for my motor club card. Surveyed the parking lot for a spot to park that would be easy for a flatbed towtruck to haul the car to the dealer (and avoid further embarassment in the parking lot). By now someone had pulled in behind me so I had to drive forward. Started the car, put it into drive and the noise was still there. Based on a 5 second analysis of hanging out the vehicle while it was rolling forward, it sounded like it was coming from DIRECTLY below the open door...not from the front or rear of the truck but somewhere more in the middle but it was a pretty quick, unsophisticated analysis. Found a parking spot on a single row that would be easy access for the flatbed. As I backed into the spot, I heard a thud when I put the car in reverse. Backed into the spot. Then decided to move forward a few feet just to see what sounds occurred. The grinding/clicking was GONE!

Drove around the parking lot with no issues and decided to try to make one more shopping stop and then head home...about 10 miles in total. It did not act up again but I am so fearful that the noises are a warning of a big problem/issue so I'm reluctant to drive it until I get some expert advise. Two other bits of information: transmission was serviced at 100,000 miles (replaced leaking transmission cooling lines...so much for sealed transmission) new MB fluid, filter, magnet. Yesterday, I engaged the LOW range for the FIRST time in a couple years for about a 10 seconds to get up a steep, dirt hill with some firewood. Truck was driven at least 50 miles after that.

Any thoughts about what would have caused this? Transfer case motor? Control module? Valve control unit? Will this have thrown any transmission diagnostic codes? My OBDII/Can reader doesn't show any codes so can only the dealer pull transmission/drivetrain codes or can any good shop analyze this?

Thanks in advance for all your inputs, opinions and guidance.

Update: Local shop (specializes in MB) predicts it the differential losing a tooth...probably front diff. Anyone concur? Any idea of what $$ I'm potentially looking at?