What kind of gain and volume settings do you have when the squeal is caused by the input jack? The ground switch of the jack connects to point F of the V1 grid circuit. So if the gain and volumes are set high, I would expect some squeal when plugging halfway into the input.
As far as microphonic IC's go, if they exist they are extremely rare. I would expect something else near they IC may be the problem.
The 5201 is a switching op amp, looks like the NJM2120 is also, the 4560 is not.