Toilets, Sinks, Showers, Tubs and Disposals - groaning pipes

my house has developed a case of groaning pipes. it wasnt that way when i bought the house 3 years ago - it may have begun after i added an automatic sprinkler system, but i didnt notice it immediately, so that is a pure guess. the groan happens in several situations: when a toilet is refilling, and also when one of my exterior faucets is used (full flow).
fwiw, we have three different brands of toilets - with different brands of flush valves (recent low-flow AS, 20 year old Kohler, and a 20 year old AS with a VERY nonstandard fill valve - so i don't think that "replace the flush valve" is the issue. I can feel the vibration in a lot of the piping in the basement, and can dampen it a bit by pushing some of the pipes pretty hard to the side - maybe it's just my hand absorbing the vibration, probably not because i've moved things around.
i'm open to suggestions.. unfortunately, not all of the basement piping is open to access ;) ..
thanks.

To see if it is the toilet fill valves, turn off the water at the toilet supply valve (one at a time) and see if the noise is still present. Bad rubber washers usually create this sound. They get old and shred. then they act like a reed instrument. Could be one toilet and the outside faucet also. Problems can come in pairs. Good luck and reply back with any questions or the solution you find.