I have a 1990 with Auto Tranny, N/A, Convertible. No mods.

In neutral, revving to the redline is smooth and results in no odd noises/vibrations.

In gear (1st), when held at high rpm - say 5000+ there is a vibration/noise. It has a cycle, like 2 seconds on, 2 seconds off. Like this - None, soft, louder, loud, softer, soft, none etc. Same behavior in the other gears, but the 'pulses' of sound are shorter the higher the gear - in 4th it goes very fast and is not as loud, it seems.

If I had to locate it I'd say it was in front or and slightly to the right of the driver. Maybe below. My guess is something in the transmission is unbalanced.

What do you all think?

 

I don't think your tranny is at fault, what happens when you get up to whatever speed the noise is noticeable and pull the tranny into neutral, does the noise still occur? If it does, then it's in the driveline after the tranny or in the front wheels. I'd check and make sure your wheels are balanced first.

 

Yeah, check the wheels, and if that doesn't help any, check to make sure your driveshaft is weighted evenly(those little metal peices behind it, sometimes they are off balance and cause that) Otherwise, good luck!

 

Excellent. I'll do that. I haven't dropped it into neutral at speed yet.