Orange Terror Bass loud crackle from volume control

[BassYerbouti]

My OTB 500 has developed nasty habit of making a very loud crackling sound for a few seconds after I move volume from zero up even a small amount after turning on and flipping the switch after its warmed up a bit. After its done it the volume control behaves itself for hours and even switching off and on doesn't get it to do it again. Do you think this might be valve related or just a knackered volume control
knob or something else?

[Bill Fitzmaurice]

It needs cleaning, with this or the equivalent. https://www.crcindustries.com/qd-electronic-cleaner/