2000 Ford Mustang Chirpping under hood

It could be a misfire, especially if it occurs at high speeds. See if you can figure out which side the chirp is coming from and check your spark plugs, wires and cylinders to make sure they are firing correctly and igniting all of the fuel. If you have a cylinder that isn't firing right it can make a chirp and cause a slight hesitation at freeway speeds. Hope this helps.