I think what you are hearing is "print through" from the master tape. According to Wikipedia "
Print-through is a category of noise caused by contact transfer of
signal patterns from one layer of tape to another."
(https://en.wikipedia.org/wiki/Print-through). If so your cartridge is in the clear, and there is nothing you can do about it.
(https://en.wikipedia.org/wiki/Print-through). If so your cartridge is in the clear, and there is nothing you can do about it.