Strange Noises Between 2,300 - 2,800 RPM

[sandhaven9]

Model (520i SE - E34 1993) Can runs quite & smooth, but when accelarating 'Only' between 2,300 - 2,800 RPM getting strange grumpling noises from rear of car, regardless of in 1st, 2nd, 3rd, 4th or 5th gear?

New exhaust was fitted recently (So That Is Ruled Out)?

When going over (Holes & Bumps) on road - No Problems - That would suggest suspesion is OK?

Whilst Stopped - Engine Vibrates Slightly When Taken Up To (2,300 - 2,800 RPM) but not as much noise as when under (Load - Driving).

Could this be an engine problem, but just hearing it from the 'Exhaust Output'?

Can drive at 120MPH and car is smooth & quite.

Thanks everyone in advance