He needs to be examined by a avian vet as soon as possible. Is it even when he is sitting or eating? Or after flying? It could be a sign of respiratory distress or even a blockage in his air ways . Please let us know what the vet says
Could be definitely a respiratory infection....did you get him checked? Any further developments?
Deb has some very useful remedies for opening up the blockages/airways. I'm sure she'll post soon those wonderful ideas.
What time does he usually do it? My budgie makes little peeping sounds when the are relaxed and resting. I'm pretty sure it's normal unless he's making those sounds when he breaths with an open mouth, and that is a bad thing.
My Milo will do little squeaks sometimes when hes excited or content with himself. He will do a little peep squeak like Faerybee mentioned and then do a little chortle and his eyes will pin. Which is telling me that hes quite happy. He will also do this as I put food in their dish and hes excited to be fed
Go with your gut feeling. If it sounds like hes in distress or the squeak is due to labored breathing I would take him to the vet. If you can see its because hes happy...then you just have a cute little squeaker.