lr-atari800 buzzing noise (loud)

[darrinsmith]

Hi
I've just installed retropie 4.4 using the pre-built image from the retropie website downloaded last week on a B+ 512K I had lying around.
So I've put some Atari 800 roms and they all load and play just fine using the emulator that was included in that image.
However I'm getting a weird buzzing noise (it actually sounds like SIO noise to me - I actually had an 800xl back in the day) and I can't seem to solve it.
The game sounds are working, but the buzzing noise makes using the emulator impossible to use. It makes this noise no matter what the emulated machine is doing ie playing a game or just idling on the note pad. I'm using the HDMI output to a Hitachi TV. It's only lr-atari800 that makes this noise, other emulators are fine.
I haven't really messed with the out of the box config apart from specifying the bios image locations. Also it doesn't seem to save the configuration on exit (when using the built in F1 menu) but I suspect this is an unrelated problem.
Just wondering if anyone else is having this problem? if so how can I solve it?
I'm honestly a bit confused between the interaction between retropie, retroarch, emulation station and the emulators, so I've probably missed something, but I kind of hoped it would work "out of the box".
<edit>
Ok I suspect this may be a processing (CPU) problem.
I'm now also having problems running C64 at full rate, although SNES works fine and the Atari runs at full rate except for the annoying noise.
I'm using a 2A power supply, which I thought ought to be enough.
I'm also running the system from an external USB as I only had a 16GB micro SD, but I don't think that should cause a problem other than slow load times, once the emu is in RAM it shouldn't matter should it?
Should I ditch HDMI and go back to composite video, I'll have to search for a cable, I don't think I have one. Does that require less processing power?
I'd prefer to stick with the B+ as I have no other use for it.
Thanks
DS