I have 4 of these. I think I have all revisions (A, B and C).

I have never had any hum (50/60Hz) problems with them. I’m not daisy chaining and using the original PSU.

Two of them had a very audible whining issue though. The whine frequency was 666Hz and its multiples: 1,33 kHz , 2 kHz, 2,66 kHz, 3,33 kHz, 4 kHz…
Although on a scope I could see whine in all of them, in the two non-problematic ones the whine was so low in volume that it was impossible to hear over the white noise.

As for the problematic ones:

In a rev A pedal all 5 electrolytic caps were bad. They read in the pF range instead of the µF range.
Judging from some oxidation around opamp U5 right above an E-cap I guess that one leaked while the others seemed to have dried up.
After replacing them the audible whine was gone.
Since these caps are all grounded you can measure them in circuit. They are smd though, so not very easy to replace.

In another one, I think rev C, I tracked down the audible whine with an audio probe and it originated right after the output buffer.
This could indicate an instability (oscillation) of the unity gain output buffer due to capacitive loading.
Depending on what pedal or amp came after it, the whine would disappear or be less audible.

Adding additional series resistance at the output removes the audible whining.
There’s already a 150 Ohm resistor at the output. It’s R35 in the rev B factory schematic which you can find on the internet.
Somewhere between 1k and 2k was enough to make the whine so low in volume that it became inaudible.
You can try some different (Boss) pedals after your Holy Grail. It needs to be a pedal which has some series resistance at its input.
Or you can unsolder the lead going from the pcb to the bypass switch in your Holy Grail and try some additional resistance there.
I used a 1k or 5k pot and dialed in additional resistance until I could no longer hear the whine.
Then measured the pot to see what resistance was needed.
You can read about other, slightly more involved solutions here:
https://www.analog.com/en/analog-dialogue/articles/techniques-to-avoid-instability-capacitive-loading.html

Another solution is adding a small capacitor in the feedback loop of the opamp at the output of the pedal.
You can read more about it in this thread at diystompboxes.com :
https://www.diystompboxes.com/smfforum/index.php?topic=127273.msg1226824#msg1226824