1989 VW Jetta 1.8 engine/transmission noise

1989 VW Jetta 1.8 F/I mileage: 110,000. Every time I am driving rather "spirited" and I wind the engine up beyond 4000 RPM, the engine/transmission noise changes from a high-pitched noise as seems normal, to a low deep rumble. In general, the car also begins to vibrate a lot more. It seems that it is only at 4000 RPM and above, and only when the car is in drive. I talked to the dealer and they said, "don’t run it above 4000 RPM (not a lot help). Anyway, the only thing that I can think of is that when it began was right after a very long road trip, say around 1500 miles. I changed the transmission fluid from whatever was stock to Redline MTL. Any thoughts or ideas would be appreciated, thank you for your time.