Turntable Thrumming

What is the frequency of the noise? Is it 33.3 one per turn of the table?

Welcome aboard John.  Have you tried turning off the motor and seeing if it stops?

Getting a test record really helps with this stuff.  It will have a track of unmodulated grooves that's made just for checking stuff like this.

It could be the motor or the belt or just groove noise.  In between tracks it sometimes goes up it seems until there is some music information.  Try taking the belt off and spinning by hand with motor off.  

Thanks for the info.  The noise seems to be around 0.5 Hz.  The sound is gone when the motor is off.  I'll try removing the platter and belt to see if the noise is associated with either of those.  And a test record should be on my wish list.

Clean (and inspect for anomalies) the belt, platter, and pully with alcohol. Remove perspex lid if applicable. 

The noise seems to be around 0.5 Hz. 

Do you mean 0.5 khz? Humans can't detect sounds at 0.5 hz.

It is cycling at about one cycle per two seconds.  The actual sound is probably 500 Hz or so.

I started working from the amp to the turntable to find the noise, and discovered that once I removed the lid from the turntable it went away.  I had played records with the lid up, which seems to have amplified the noise. 

That would be platter rotation speed. Your platter is rubbing something or it might be warped. The only other possibility is a bad bearing. Both are warranty issues.

How do I check for either of these?

The lid is acting as an amplifier of acoustic feedback from the room and feeding it back into the system.

Remove perspex lid if applicable.

once I removed the lid from the turntable it went away.

My guess is that is normal motor noise. It stops when you stop the motor. And with the lid up it’s resonating and amplifying that hum.  It would do that less with the dust cover down which is where they should be while playing IMO.

It’s why I moved my motor to a sub plinth. Try taking the belt off. Place the needle on the record and turn on the motor. If that’s when you hear it, it’s the motor.

Thanks for all of the advice.  I'll remove the belt and see what happens.