This post expands upon a toot I posted a few days ago.
As I suspect many GMs do, at least when running sandboxes, I have rumour tables for my current Héros & Dragons campaign. Here is a sample from a recent session:
Men from the Sea Principalties have been mugging merchants in the Harbour District.
Notice how this rumour is assertive. No “some people say”, no “may”, no “there is a rumour”. Just a fact as you may overhear it in town.
My tables usually do not include the source of the rumour. Sometimes it is obvious, sometimes players may have to ask around to find the source. I may include the name of the person reporting the rumour, as in “Sir Ramald says orcs have been seen crossing the river two days north”; in this case Sir Ramald himself may be the origin of the rumour, or he may just have been the one to spread it. The players won’t know until their characters ask him.
Finally, my copy of the table includes additional info: whether or not the rumour is true, of course, and anything related, like who is behind the muggers or where the orcs are going. Even false rumours have additional info: who is spreading the rumour, why, what the truth behind it is, or how it might lead to something unrelated at first sight.
My main point: make your rumours assertive. If the players are interested, they may investigate and find out the details. If they are not, well, they just have heard some random tidbit in town.