A likely cause of such a noise is deterioration of the driveshaft center bearing pillow block, which will allow the driveshaft to flop around.

My center bearing and pillow block were replaced on warranty at about 40K miles because the bearing had spalled and was making a growling noise.

At about 80K miles my 190E 2.6 developed a squeaking noise from underneath. It was most noticeable at about 15 MPH. With the car on jackstands and the catalyst heat shield removed I was able to reach up, grab the driveshaft, and wiggle it around, and then saw that most of the pillow block rubber was gone.

Detonation is most likely to occur under low rev/high load conditions and will be more prominent at high coolant and/or ambient temperatures and won't sound like its coming from under the car.

Duke