Recently going about 70mph I punched the gas and as it downshifted there was a LARGE bang(clunk)? This was the first time this happened. I'm leary of doing any more hard downshifitng as it seems to be continuing. Car had 77K when I bought it and now has 112K.
Trans serviced twice last time at 90K.
Front flex disc replaced at 78K.
Could this have anything to do with "Kickdown Solenoid" or the "Vacuum Modulator"???

Not positive... but the clunk could be a symptom of worn transmission mounts, or differential mounts.

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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)