So this has an interesting twist.
Scott brought over a pair of Memory Lane 2s that he owns. One of them was dead quiet, and the other worked great, but after a little while this annoying background whine would come on, like one of those European police/ambulance sirens. The whine was clearly synced to the LFO, and would go up and down faster as the Speed control was advanced. It was also present in bypass mode.
At first I thought that this was merely a case of the clock signal leaking through due to a mis-setting of the balance trimmer. (Bucket brigade chips have two parallel paths internally, and their outputs can have a "clock-bucker" effect if balanced just right to cancel out the clock signal). I wrote to Diamond for some info to identify which trimmer adjusts balance (there are 8 trimmer inside), and repair person Brian Fecteau was very helpful in providing two annotated pics of the inside with trimmers relevant to several functions circled, and adjustment notes provided.
I dickered with the trimmers, to no avail. I wrote back to Brian, indicating lack of success, but this time provided a little more information, I noticed that the chassis indicates 24VDC for the power jack, but the wallwart Scott provided is 18VDC (though he said the adaptor came with the pedal). The pedal uses three voltage regulators to bring the external supply down to 15, 12, and 5 volts, successively, and are visible here as the three objects leaning against the chassis, on the upper left with metal tabs. Such regulators require an input of at least 2V more than they output, so I was wondering if the wallwart might be the source of the problem. It is not uncommon to include diodes to protect the pedal against use of the wrong wallwart, and these could bring the voltage the regulator sees close to, or even below that 2V requirement. But no, when I touched my meter probes to the relevant pins on the 15V regulator, it put out a nice stable 15V. It is normally the case that supplies are "down-regulated". That is, the external supply is regulated to provide one voltage, then additional regulators successively drop that regulated supply voltage to lower ones. I figured if the 15V checked out okay, then the rest would be fine.
BUT, I also mentioned to Brian that the whine didn't really come on for about 5 minutes, which suggested to me a problem with heat or a bad capacitor somewhere. All the discussion of regulators, and the mention of the "warm-up time", triggered a recollection on Brian's part. He mentioned that in an earlier run of Memory Lane pedals they ran into problems with some of the regulators; specifically the ones used for 12 and 5 volts. It seems that the tabs on the batch they received had unusually thin heat-sink tabs. He sent me a photo of what to look for, and sure enough, Scott's pedal had the offending regulators. The ones you see in the pic below have "normal" heat-sink tabs. The ones they had naively installed in the run of pedals that Scott's came from were less than half that thickness.
Anyhow, this weekend I'm going to replace the offending regulators with new ones I have, and hopefully the appropriate heat-sinking capabilities will coax the pedal into blissful quietude.
Many thanks to Brian from Diamond. And also to Scott for trusting me with his pedal and providing the opportunity to learn something about consistency in commercial parts that I would have never suspected.