Press the key at different dynamic levels and see if a softer or louder dynamic creates the squeak. If it is a louder dynamic level that causes the squeak, you might actually be looking at a buzz instead. Check the soundboard and hammers for any debris, such as wrappers, coins, paperclips or even paper that may be causing the noise.
Insert a soundboard steel between the sides of the keys, and attempt to dislodge anything that may be stuck between the sides of the keys. Even a small splinter will cause squeaks.
Apply Protek to the balance rail located directly inside the piano at the bottom of the hammerhead shanks.
Dust the inside of the piano by using a clean cloth and a vacuum cleaner with a soft bristle attachment. Vacuum the piano key heads and the soundboard to remove any debris.
Sand down the sides of the key by pressing down on the keys to the either side and using a fine grain sandpaper. Sand just enough to smooth out the key and prevent future squeaks.