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to my knowledge and experience, only three things make noise in a TDS scope:
The speaker on the A12 board (the front panel with knobs...)
an arcing High voltage lead from the HV transformer to the CRT
you can remove the A12 board by removing the front cover (look in the service manual) and simply unplugging the connection to the CPU board (top). You turn the scope on using the push button on the right side of the CPU board (it's the only button on the board). If the sound has gone away, you have a speaker problem.
Otherwise, If the HV lead has become loose, then you need to follow the directions in the service manual to get to it. They include removing main case and front cover, the CPU board, the panel underneath the CPU and finally the front panel and cover for the CRT.
Make sure you ground the lead from the HV to the crt to the crt attachments to avoid getting a nice (but probably not lethal) shock. pinch the leads and re-secure the HV lead into the crt hole and make sure it's firmly seated. The rubber gasket should be nearly flat when installed. BTW, this hole is the connection to the CRT capacitor that takes the 17,000 volts and is used power the screen mask that accelerates the electrons from the heated cathode towards the front of the screen.
If someone put in a replacement LCD, there will be no HV board and you would have a small circuit board in it's place - pretty easy to tell.
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