The departure areas in the international terminal are often very quiet. Once you clear security into the United domestic terminal, follow the signs to the international terminal and walk around. (Or, I suppose you could just go upstairs after clearing customs and clear security in the international terminal itself, but if the TSA gives you grief for being in the wrong terminal, just clear in terminal 3.)
Depending on the time of day, you'll see a number of gates with no planes, and it should be pretty quiet. In general, the design of SFO's international terminal promotes a quiet, almost serene atmosphere, because the gate waiting areas are on a different floor from the main corridor. This design causes the noise from hundreds of people and dozens of PA announcements to be contained to individual, isolated areas instead of spreading and combining into the awful cacophony that assaults the ears in most airport terminals.
Edited to add: well, if you have 5 whole hours, and you aren't absolutely smashed from the long flight, you might want to seriously consider the advice to take BART to the Embarcadero or downtown SF, and just stroll around. SF is beautiful!