@artr
I had a knocking/scraping sound on my 34 a few years ago, like your 34, mine had been stood for a few years and the belt had gone to goo. The knocking sound came to light after replacing the belt, when laid on its back it was quiet.
Inside the flywheel there is/was/should be a washer with...
Here is the relevant section from the diagram on page 20 of the manual dealing with the meters, the description is on page 28.
Incidentally the manual on Hi-Fi engine was uploaded by me.
If you are still having no luck try this:
Firstly, double-check your connections between the M-30 & 34.
This checklist will use the submix section for playback.
Going back to your earlier post; the fault above would suggest that unless there is a fault with channel #1 on the 34, that the record select button for channel #1 on the 34 was out, thereby keeping the channel in playback mode.
If I get the chance later today, (currently 1:00am here in the UK)...
In all probability the one you downloaded is not the complete manual. The full manual runs to 100 pages which includes fold out pages.
If you want the full manual you can download it from my website here; File Store as a PDF it runs to 88 pages.
All the fold out page sections were scanned...
If you keep it set to monitor, you will be able to see the levels going out to your preamp.
As it stands with the current settings the submix is as you say "out of the loop".
Just asking; the manual you have it Tascam's original printed manual, or is it a download?