“Why is it some discs (like the new CD I just used to install Toast) make the drive sound like a lawn mower engine, while music CDs all seem to be silent?”
The sound is probably caused by the thickness, weight, or the balance of the disc being slightly out spec in one way or another, causing it to rub against some part of your drive mechanism. It’s generally nothing to worry about so long as the data is readable.
Audio discs are manufactured in (usually) very large pressing facilities, and they quite often have tighter manufacturing tolerances due to the wide variety of audio CD players in the field. CD ROM discs are generally inserted into ‘tray’ type drives and are usually made in significantly smaller runs than most commercial audio discs, thus they are sometimes manufactured at smaller plants with looser tolerances. Furthermore, many installation discs only get used one or two times. It’s a small, but significant cost savings to do these at cheaper plants.
Exacerbating this issue is the rate of spin for a CDR, which is up to 40 times faster than the rate of spin for audio CD’s. Any minor inconsistencies in a CDR’s construction can be made apparent by this increased inertia.