Degreased or grit in the headset? Front wheel bearings loose/dirty/degreased? Loose or recently rebuilt front wheel (spokes will creak a bit after rebuild, until they settle)? Front axle nuts incorrectly tightened (possibly front axle not settled into fork dropouts before tightening - happens sometimes if put in on a workstand)?
Some of the above are serious, others just need minor correction. Others will be able to suggest other possibilities I'm sure.