my mac(2011) 13 inch running windows 10 has a weird fan.
it turns on when in regular use like browsing online or word processing, having multiple windows kicks the fan on in a long but almost silent hum. at random times it is high pitched and very loud. is there something wrong?

The fans are controlled by thermal sensors. If the motherboard and other stuff (power supply, etc) are dusty, this can exascerbate the problem of high internal temperatures. Take the cover off, and vacuum out the system (unplugged preferably). For hard-to-get at nooks and crannies, you can also use some compressed air in a can to blow the detritis out to be vacuumed up.

Sources of heat in a system? CPU, RAM, power supply, disc drives are prime candidates.

Rubberman is correct.

On the 2011, 13" macs, you can remove the bottom by unscrewing the tiny screws along the base plate. There are 2 sizes of screws, so keep careful note of what size screw came out of what hole. Pull the cover off with gentle force...use a flat head screwdriver to wedge the cover off. There will be sticky stuff holding it down. Clean the fans of dust then put it all back together.

And thanks for your post @fallout4player. This may help others as well. I am giving you an upvote for your post.