MY first suspect would be in the exaust. That it may be leaking well before the muffler(thus the loud engine sound). My other suspect would be a syncro gear faulty in the transmission(from the gear shift sound you mentioned). Without more information that is the best I can say. Hope you find what’s up.

since it has a v8 (119 motor i do beleive)the camshaft oil tubes become faulty, one of the end caps may have popped out and does not deliver enough oil to the lifters and the lifters are actually making all the noise common problem with those engines ,simple fix for about a 1000.00.good luck take it to a benz shop.

this is not something you could repair yourself…
go to service… it’s probably a shift problem… not necessarily with the gears themselves

It could be one or more of many things. The fan belt or fan bearing may need replacement; the exhaust could be rubbing with the shaft; well it would be best to take it to a mechanic.