Soph, I do that when I'm gardening, just put the words and/or music somewhere fairly safe from flying debris. The motivation to recall correctly increases as I wish to avoid the annoyance of leaving my precious earthworms to go have a look at what comes next. Sometimes I find myself trying to imitate bird songs (an octave or more lower--I'll bet your sweet treble can get most of them at pitch) but it's a challenge with only one set of vocal cords when birds usually have two.

Meebs, yes, there is usually a predominant pitch to the drone, but I'm very sensitive to upper partials which are too cacaphonous to my ear when generated by electric motors. Gas motors offer only a slight improvement, which is offset by the greater volume. Given the many musical interests I have, [did you know that teasel seed in a plastic bottle makes a great shaker? or that a length of septic tank drain hose with one end covered like a drum makes a fabulous shhhuk-sshhhuka when stroked with a hairbrush?] I doubt I'll be investigating which amp ratings, rpm, horsepower etc. are the least bothersome to me.

Cheers!
CH.