Vibration maybe?. I recall seeing rubber O-rings that fit around preamp tubes to limit vibration/microphonics.

Edit,.. these.. https://www.amazon.com/dp/B07WS9SY96/ref=sspa_dk_detail_3?psc=1&pd_rd_i=B07WS9SY96&pd_rd_w=cJYGq&content-id=amzn1.sym.3be1c5b9-5b41-4830-a902-fa8556c19eb5&pf_rd_p=3be1c5b9-5b41-4830-a902-fa8556c19eb5&pf_rd_r=R758XXAB9220679WF1KW&pd_rd_wg=fBdm7&pd_rd_r=a7a1f62c-0c86-497c-a48a-8f1f1dbd1b3e&s=musical-instruments&sp_csd=d2lkZ2V0TmFtZT1zcF9kZXRhaWw&smid=AOXF6RRHAABMW&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEyWjRNT0hLVzdQWFNLJmVuY3J5cHRlZElkPUEwMzAwODkzVVQ2QlMwMjc4NUQ5JmVuY3J5cHRlZEFkSWQ9QTA4MzkwNjIxTDE0WTY3V0RRNzNFJndpZGdldE5hbWU9c3BfZGV0YWlsJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==