Squeaky seat &/or post?
 

Odd noise from a fixie - thought initially it was noise from the pedals, now believe it originates in the seat/post joint or seat post/frame joint. Tightened both securely, but the squeaking continues, probably from the slight continuous rocking on saddle. Aluminum frame, carbon seat post, seat & seat post designed as a unit therefore not an issue of fit.

Does anyone use a lubricant at these joints? Suggestions?

I had this problem, and solved it by removing the seat post, cleaning it, and re-greasing it with a thick grease.

Oh, but if the post is carbon, you may need something special here, a special lubricant or something. Normal grease may not be OK. See your manual on this.

Use Tacx Carbon Assembly Compound or equivalent.