Why would capping one sprinkler zone cause my toilets to whistle?
Whistling toilets after plumbing alterations which occurs continuously except when flushed most likely indicates that debris was dislodged and is now affecting proper seal of the fill valves.

Turn off the water to each toilet, disassemble each fill valve, and give them a thorough flush by cracking the supply valve and letting water flow through, rinsing the washers and internal components. Heck, now is as good a time as any to go ahead and rebuild them by replacing all internal gaskets/washers.