Just the other day, I started noticing that under full acceleration, it sounded almost like the car was peeling out when reaching 2500-3000 RPMs (without the screeching that peeling out would normally make). And just tonight I applied full pedal to get the car up a little hill and it made that same almost grinding/peeling out noise from distinctly the rear drivers side of the car. The engine reacts like the drive tires are losing contact with the ground...

Any idea on what this is? Is it a bad idea to drive the car with this happening? Any insight would be much appreciated!

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1987 300SDL 271,000
1999 E300 Turbodiesel 155,000ish. (Dad's)