Check your chain system, not only on the chain, but also on the chainrings, cogset, and derailleur system. Sometimes this part will also make noises because of rust.

Drip the WD-40 bicycle lubricant on one spot of the chain and rotate the foot pedal until the whole chain is lubricated.

Only use WD-40 that is specifically made for bicycles. Using normal WD-40 can damage your bicycle chain.

Repeat this process until the chain no longer squeaks.