turntable " thump " sound when not running

Hi I have a question that’s been in the back of my to do list for the longest time so I decided to join the forum after reading so many postings over the years from this legendary community. :) I have a thump sound the same way it sounds when dropping the stylus on a record. it occurs about every 10-20 seconds or so regardless if the turntable platter is spinning or off. I am guessing its electrical? what causes this? I have a pretty decent setup yet modest compared to most of you guys in here and I think I shouldn’t have to deal with this thumping sound.  By the way this sounds only occurs on the phono.  the cd, tape, etc doesnt have this problem. 

Adcom GFP-750 preamp
Adcom GFA-555II amp
Jolida JD9 II
Music Hall 5.1SE TT (Sumiko Blue Point no. II)
LSA-1 Statement speakers

I have the turntable power supply adapter plugged into the back of the preamp power ports.
Should I move it to a different outlet? Maybe I just answered my own question.

Only reason to plug into the back of the preamp is that you power off the turntable, when you shut off your preamp.  It is better, in general, to plug the turntable into a separate outlet.  Your Adcom equipment is in the 25 years or so age range.  Rule out that there is an issue with the back outlet.


obeerbongkenobi (clever screen name)

One thing to check for sure is that your preamp (or any other audio gear) isn't on the same circuit as a high-load cycling appliance.

A refrigerator or AC unit for example can draw huge amounts of power when starting up the compressor and also cause issues when they power off.  The reason you would only hear this thump when you have the phono selected versus other inputs is that the gain is so much higher than the line-level sources.

This would be my guess as to what's going on.

Have a look and report back

Try disconnecting your turntable from the phono pre and short the individual phono inputs (not to each other). If it still thumps, it’s likely the phono pre has a bad tube or is going bad.

When I don't play my records, my preamp is on MUTE.
When I lay down my needle on record surface, my preamp is on MUTE
When the needle starts surfing the record i UNMUTE my preamp.
When I want to pull up needle from my record, my preamp is on MUTE.

If you don't have MUTE switch, use input selector to switch to AUX or any other line input.

Thanks for all your feedback everyone.  I ended up unplugging the Turntable power supply from the back of the preamp and plugged it directly into the wall outlet and then turned on everything so i could listen for the sound and it was still there.  So then I started to jiggle the ground wire that runs from the turntable base to the Jolida Phono preamp and the hum would come and go.  then i noticed the end from the jolida pulled out completely and sound went away.  neither ground hum or the thump sounds!!.  I detached it from the TT as well and just took out the ground wire altogether.  Strange.