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I second what JR said.^
Given that with the 3RD IF pulled and the signal injection after that, that the interference remains. The problem is most likely noise in the B+ or HV arcing. There could be issues in the stages between the video detector and the CRT, but it is not as likely.
Since you are getting sound interference too, here is an idea to save you some time....Pull the H output tube or damper (to stop supply of HV) and see if the sound interference goes away after it has had a couple of minutes to run without HV (you obviously can't check picture with no HV).
If disabling the HV stops the interference then the issue should be in the HV or CRT grounding. If the noise remains without HV then check the B+ lines with a scope, and if those are clean use the scope to look at the stages between the injection point you were using and the CRT video feed.
You may have AGC issues, but if as you say the IF is passing signals properly (with the exception of the mystery noise), then I'd ignore the AGC until the noise issue is fixed. The AGC appears to only affect the tuner, 1ST and 2ND IF...So given you can inject after the 3RD IF with the 3RD IF tube pulled and still get noise, I think it is safe to say the AGC is not your noise source, and can be ignored for now.
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