Cruisin' on my Bluebird
(edited) Squeak detective Pete here. Squeaking is not acceptable imo and all squeaks matter! Why? Drives you & others absolutely crazy and/or could be a sign of trouble.(see recent failing cranks) Most likely it is simply metal to metal contact somewhere. Many spots. My go-to last resort to find the offending and offensive sound is a can of PB blaster or any good spray lube "penetrant". Start with the seat, then the crank/sprocket area then the front and rear axles nuts/drops, the stem/headset interface and handlebar/stem clamps/nuts and entire headset. Hopefully greased the BB and headset cups before you installed them. The seat and crank are the primary suspects, so I would start there. Spray heavily one area at a time then ride it until you isolate the intruder and expose it for what it is. Then you can grease that area's metal to metal areas. Pedal cups/ends and axle threads have also been a culprit, but spray lube will find it. Works every time.