tight rollers might be the cause but this is usually accompanied by scraping or some other abnormal noise while the carriage is traveling up and down. michel's suggestion is more common as i've see quite a few valve issues lately. A faulty safety valve can cause this exact symptom.
One trick to see about this is to raise the mast all the way up, then lower it, and see if it does it anymore. If the problem goes away then it is most likely the safety valve. If it does not then i'd look elsewhere. Still you will probably be breaking the cylinders down to see what the problem is.
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