I haven't used the rubber tube method, but I have used the screwdriver to the machine and ear method. It's loudest at the top, but I can't pinpoint the exact location.
CD,
The squeak happens with or without the needle plate on.
Miriam,
Could be, it's loudest on the left end of the machine. But I can't pinpoint it.
As I said above I've oiled everything that moves repeatedly. If it's a lack of lube somewhere, it's a weird place.
I've got to put the needle bar back in, then I'll get back to the hunting.
Joe