I saw a show where a guy took his pocket knife and was gratting it against a walnut, making it sound like a squirrel was eating on it. That seems like it would be a good idea, to at leaste get a squirrel curious enough to come looking if you were hidden real well. Even if it would pull the squirrel back around a tree to the side where you could get a shot at it, or pull them up out of a nest.
How I tend to do it is if I know Im in an active squireel area and Im hunting for them, Ill pick a good spot and sit down for a bit and stay hidden as best as I can and let the woods "settle back down". Sometimes it takes a few minutes, other times it just doesnt happen. If that doesnt work Ill ease as slowly and quietly as I can through the woods. Pausing here and there behind a tree for a few mins. Didnt do much squirrel hunting last fall but the fall before I made out real well with these two methods (26 squirrels, with my "limit" several days) That and Ill wear camo BDU's and a cammo t-shirt, and even use no scent. Dunno how well they smell but if nothing else it gives great practice before deer season.