I have a situation where a loud noise comes from the plumbing in the wall after you flush and the water fills back up. The noise starts after the water is full and it shuts off. If the toilet is used often the noise/ vibration does not happen. But if it's only used once in awhile as soon as the water shuts off is sounds like a horn. Is this have to do with the pressure reducing valve? Too much pressure?

It sounds like air in the system but you could try putting a new washer in the ballcock as the one in there now could be worn

Thank you

Sounds like defective seal in fill valve. Which type do you have?