It could also be dust on the heatsinks and/or fan that spins that makes the fan not spin as fast, if you don't have compressed air you might want to try giving it a seriously good blow through some of its vents to see if any dust comes out.
WARNINGS!!!!
1. This will be messy, don't do it in your living room. Step outside for a minute and make damn sure the door is closed behind you before you do a good puff, you could get a BIG cloud of dust and I've found that tends to make spouses/children/friends less than happiful to be in the immediate vicinity of.
2. Try to have your mouth as dry as possible, do NOT spit in to your unit! I've (cough-cough) heard that smoking marijuana in addition to making everything betterer also dries out your mouth so I guess if I were to be doing something like that and I hypothetically had access to said substance I would probably use some to prepare in advance and to keep my components dry.
3. Do not put the unit's vents up to your mouth until you have taken a mighty inhale. I have seen way too many amateurs put the device up first and then inhale for their power blow only to be shocked that they just sucked a great deal of dust and muck in to their lungs. Do not do this. Take a deep breath BEFORE you put your mouth to the unit.
4. Please don't tell me that the fans spins so it can't be dust, it still could be dust even if the fan spins. Many components in there are passively cooled, meaning they just got a little heatsink on them to shed their heat. When dust gets on those, it's like a nice little insulating blanket that helps keep them warm...but they're not supposed to keep warm they're job in life is to get cold and suffer.
5. Best of luck, just thought I'd try and make my instructions bullet proof.
