1987 635csi M30 engine sound changes around 2800-3000rpm

[CThatch]

Have done all regulator maintenance on the engine including, plugs, wires, oil, oil filter, air filter, valve adjustment, distributor and rotor.

Car seems to run well, and power seems good but I notice a distinct change in the sound of the engine around 3000rpm.

Not sure if this is normal I've not done this much work to an m30 before and am used to Mercedes engines.

It just has a distinct louder scream to it so wondering if this is normal or if anyone else has experienced this before.

[1986series6]

More details needed. Auto or 5 speed? Full or easy part throttle in what gear? Engine or exhaust sound change?
Does the sound change only between 2800 and then stops after 3000? What kind of scream or sound. Could it be a screeching
or a high pitched whistle type sound, or the whirring sound of the engine fan?

Will try & help best we can.

Bob V