Intermittent squeak when pedalling what could it be?

[Brooks]

Try taking off the crank arms and apply at bit of grease to the hole. A smear of grease on the pedal threads would help as well.

 

[roley poley]

Had a mudguard stay that contacted the carrier frame and gave an untraceable cheep for weeks but solved with a bit of stay bending

 

[Landsurfer]

Squeak ,,, 500 mile squeak ....replaced the bottom bracket .... greased and torqued the cranks, greased the hub bearings. Fretted and wondered.
Sprayed the plates on my shoes with WD40 ....
Silence .. !!!