My Weihrauch HW100 (which is factory regulated) used to do that sometimes if it had sat for a few weeks. Usually quit after a dozen shots or so. Started leaking soon thereafter, but it is an '04 model after all. A teardown and light lube of the washers and orings in the reg would probably fix it. It did on mine.
Or maybe it's an FX thing? Could be since mine only done it after I installed an FX airtube onto it!
....just pulling your leg