95 Passat VR6 whining @ 2.5k - 3k rpm

The 95 Passat is running excellent except that: When I accelerate on highway and the rpm is between 2.5k - 3k, it whinnes very loud. If the rpm lower than 2.5k or higher than 3k, there is only little noise. I wonder what is the problem or is it a problem?
BTW, in last 6 months I did the following repairings:
1. Replace the leaking Air Intake Boot;
2. Replace the two leaking Coolant Pumps;
3. Replace the malfunction Idle Stablizer;
4. Replace the seized Tensioner.

Thanks in advance.