@lewm , worse. I'm already maxed out on the tranquilizers. I'm trying not to get too far into the Islay.
@tvad , that will work. As Lew suggests that will move the frequency the dust cover vibrates at up higher. There is a confluece of issues at work here. It is not just the dustcover vibrating but pressure waves under the dustcover are exciting the tonearm directly. You can bang on and vibrate the plinth all you want and you will not be able to hear it with a record playing. That path is cut off. Heavy bass drum gets the cycle started, the pressure waves under the dust cover vibrate the tonearm at a frequency it is sencitive at for some unknown reason, generating more signal at the subwoofer and around we go. Sealing off the opening at the bottom of the dust cover stops it. So it is a Helmholtz effect. The next question is why a correctly set up arm/cartridge combination should be sensitive at 24 Hz. It may also be sensitive at other frequencies. 24 Hz is just the one being generated by the dust cover. For fun I am going to install a different cartridge this weekend tp see if it changes anything.
@mrklas , I have always used a dust cover during play. Even at the age of 13 I had my TD 124II mounted in a cabinet under a hinged dust cover that I made myself. I have used a conductive sweep arm since the late 1970's. Records collect static electricity very easily. They will actually automatically charge themselves! The paper label donates electrons to the vinyl, the vinyl goes negative and the label positive. You can see this yourself. Wool is at the very top of the triboelectric series. If you take some fine wool thread, tie it around the end of a pencil and leave a two inch tail you will have made a very sencitve static electricity detector. Take any record out of it's cover and wave the tail over the vinyl and it will be strongly attracted. Wave it over the label and it will be repulsed. Dust and pollution are for the most part positively charged and the record attracts them just like the wool thread. 20 minutes out in the open is plenty of time to collect all sorts of things which your stylus will happily grind into your records. It must be a problem because a large proportion of audiophiles spend a lot of money on cleaning devices, fluids and chemistry. All I use are a conductive sweep arm and a dustcover. I never have to clean my records and I hardly ever have to clean my stylus. All I have is a Spin Clean in case somebody brings a dirty record over. I do not buy used records. If I did I might have something more ellaborate. The trick of having a clean record collection is not to let them get dirty in the first place. Then you get, "But, they come filthy from the manufatcurer." Other than some incidental surface dust they do not. I know this because after playing many new records I have stared at my stylus under magnification and there is never anything on it. No dust, no goo, no mold release, no plasticizers, nothing. Don't beleive me. Do it for yourselves.
I will report back after changing the cartridge.