Might be the exhaust pipe banging on the cross-member, the 300SE is notorious for this when the motor mount sags a bit. The torque of the engine when in gear would move the exhaust down closer to the cross-brace.
Easy to check, just turn left and accelerate, if it makes more noise than turning right, that's it. Or you can just look at it, there should be at least 1/4 inch clearance between the exhaust and the cross-brace (near the steering idler arm).

DG