Geez, seems like the Merc is falling apart now...
Car has 133K on it, at certain speeds the car vibrates for a second then stops. It seems like when accelerating on the highway this happens at 45-50mph. The vibration is audible too and sounds like a deep tone buzz. I think it may be doing it slightly at city road speeds but it is not that noticeable.

The strange part is it does it just for a second then it is fine.

Any ideas?

 

check the transmission fluid. It may be low. also the ignition module may have an issue. if it's causing misfire or backfire for short duration, that may cause that sound. I don't remember what year yours is, and I'm too lazy at the moment to track that down, but depending on how old it is, there could be some sensors that may be a little out or it could be a timing chain tensioner that's gone and causing some valve blow-by at a certain torque envelope created at that speed.

can you tell what part of the car the sound is coming from. That might help some in tracking it down.

 

It sounds like the drive shaft/transmission area, obviously I can't tell for sure since I'm in the vehicle but that is my guess. I will check the fluid...

Year is a 97

 

Tranny fluid was fine. Car was on a flat surface, engine warm, shifted through gears and the fluid was at the top of the hatch markings.

 

the fluid was a nice red color and not amber or brown right? (if amber or brown, the fluid is bad and there could be internal transmission problems.

it could also just be the overdrive slipping (may be time for a trans rebuild - soft parts - clutches, filter, and fluid). it could also be the torque converter having issues at lockup and slipping.

 

Color looked ok to me.

I was afraid of a tranny rebuild required...guess I'm going to try and sell it as is. The car isn't worth the money it costs for a rebuild.

 

check the u joints you may have one seized up

 

I supect that what you are experiencing is torque converter shutter - that's what the dealer called it.
I have owned a 93, 95, 97, & 03 CV and a 99 MGM. I experienced torque converter shutter in all of these between 25K & 30K miles. The first indication was usually a shutter (vibration) at about 45MPH while accelerating, for some reason mostly when coming down an on ramp to the main highway and about the point of shifting to OD.. As best I can describe it, the shutter was somewhat like momentarily going over Caterpillar tread indentations in the pavement. This vib also sometimes occurred during slight acceleration at highway speeds but was usually very brief - only a couple seconds or so.
The fix was to flush and replace the transmission oil. I believe Ford required a change at 30K intervals. This was then good for another 20-30K. However on my 03 CV, I had to have the trans pan pulled and replace the filter after another 15k.
Pressure flushing would seem to me to not get all the old oil out but apparently for the most part it does. It's possible that you could still manually drain the torque converter on the 93 but I may have it confused with an 89CV I once owned.
In none of these cases did the trans oil ever smell burned or had it changed color.
Hope this helps