Back in the old days we'd just smack the side of the case....
Had a "budget" EQ like that once.
Bought it second hand for a B rig.
Every 4 or 5 shows it sounded like rice krispies. A sharp smack in the middle of the back cured it for another period.
During yearly maintenance I normally pass a vacuume and compressed air through everything (with covers off) and have a close look at all the connectors.
Guess which XLR socket wasn't soldered to the circuit board......
In all fairness, the other 3 were soldered in just fine.
Friday afternoon assembly ??