That should be a two-tone beep. It's the standard BBC power-up sequence.

In mode 7, 1 byte of video memory corresponds to 1 character on the screen. So it looks like you have a problem with address line MA6 from the 6845. If you can borrow a logic probe, look at pin 10 of the 6845 (look at 8, 9, 11 as well, in case I've miscounted) - there should be a square wave (pulsing) there, and then trace it through IC36c (again, there should be pulsing at pin 8 of this chip).