I have noticed lately that the guest toilet has been making a high pitch noise,without flushing and filling just out of the blue, at the same time the water pressure is low. I'm hooked up to city water. I live in a one story modular home with two baths. Suggestions?

Sounds like either a leaking fill valve or maybe your supply pressure is too high.

or a leaking flapper along with debris in the fill valve ??

replace fill valve

I'd start by changing the flapper, it is cheaper and easier than changing the fill valve. Also, it sounds like your flapper is letting water leak down to the level where the fill valve kicks on. The fill valve could also be noisy, but if it had failed completely the water would rise and run into the overflow rather than switching on.

When you change the flapper be sure to wipe the debris off the flapper seal. If you have an older toilet with a brass Douglas Valve, change out the flapper (Douglas) valve.