Clicking noises in steam heat pipes??

I live in an 80 year old apartment building with steam heat and our boiler was recently replaced, but all of the pipes and radiators are existing. Since then, there is a constant clicking noise coming from one of our vertical pipes every time the system turns on or off. It lasts for at least an hour each time. I don't think it is water hammer, it is not as loud as that. Sometimes it clicks about every 5 seconds, and sometimes as often as every second. The plumber who installed the new boiler has been trying to fix it, but nothing is working and he seems to be out of ideas. Does anyone know what could be causing this??
Thank you!

click

Apparently it stops after a time and then starts again when the burner shuts off. If so, then it is an expansion/contraction noise, possibly because the new boiler heats up faster, and possilby cools down faster if the unit is much smaller than the old boiler, than the old one did so the sound is more noticeable.

(Most likely -->)Could be an undersized header coming off the boiler, allowing 'wet' steam to enter the piping.

Any thermal expansion should be short lived. 'Ticking' initially after boiler comes on, and after it goes off would be thermal expansion, but not while the boiler is running.

Older boilers were much larger physically, and allowed for water to separate off the steam within the boiler itself. Newer boilers are much more compact and require an oversized header with at least 24" of vertical rise to allow the water to separate to give good quality steam.