When I launch Pd and set the DSP going (even if there is no patch open at all) the Mac Activity Monitor shows 'coreaudiod' suddenly jump to using 20-30% CPU and I notice a high-pitched whining sound coming from the computer. Other audio software does not have this effect. Is this normal? If not does anyone know how I can fix it?

Macbook Pro OSX 10.13.6 with Pd-0.51-4

@beem I think you can safely assume that it is not normal.
Have you patched a feedback loop for Pd in the Mac Audio settings app?....... or patched some other connection to some other app that reverts to a feedback loop for Pd when the other app is closed?
Or set Pd AudioSettings to connect to audio pins that are connected together in a soundcard driver?
Try setting Pd Audio settings to "none" for input and output first I think and then try to find the cause.
David.

@whale-av thanks for your reply. I don't think I have a feedback loop as I only have the standard input/output setup of built-in microphone and built-in speakers with the normal internal soundcard. If I switch the input/output to none in the Pd settings the 'coreaudiod' does stop gobbling the CPU but beyond that I don't know what I can do to sort out the root cause.

Just in case anyone is having similar problems here's what I've done:
I had an old USB 'iMic' kicking around (USB mic/line in and jack output) which I've plugged in. Selecting that for input and output instead of the built-in mic and output keeps the coreaudiod activity down to a more reasonable 6%. So I assume something is going wrong between Pd, the coreaudiod driver/component thing and the built in mic/output.

@beem It really sounds like you have feedback between the internal mic and speakers, try turning down the mics volume.

@oid I don't think so as Pd is not running a patch.
There could be another app........ soundflower for example.... creating a connection.
Actually I suspect a messenging app.... zoom, discord, Skype......... they all hook into speaker and mic and process audio between them to cancel echo and noise....... maybe look that way as it is a new and very unusual nasty......
David.