The distributor is less than a year old. If I start the engine up after it has been driven (still hot) it will hum for a minute or so. Oil pressure is around 30, where it has been for as long as I can remember. If I touch the dist. cap, the vibrations match the noise I feel. I don't think it's my main bearings because I think I'd hear that all the time. The pinion on the end of the dist. looked fine when I looked at it in August.

 

Is the rotor hitting the inside of the cap? Maybe cap's on slightly crooked somehow.

 

Doubt it, the noise goes away a minute or so after start up. Rotor and cap look unharmed.

 

Check the condition of the distributor gear teeth (pull the distributor out). See if there is play in the distributor shaft bushings while you have it out.

 

The teeth look fine. I think the bushing may be bad. I didn't feel play but it wouldn't surprise me if it was defective anyway.