Running water sound in baseboard heat

Almost 2 years ago, we had a new oil fired furnace installed. Last fall (roughly 1 year after furnace installation) we started hearing running water when the heat would come on. We would only hear it in the master bedroom. Our service guy (the one who installed it) fixed it at no charge. Now it is happneing again, only a little worse this time (we hear it in 2 of the bedrooms). What causes this? Is it something a "do-it-yourselfer" can fix? It is a ranch type house with only 1 zone throughout.

Thanks!