Battery may be aging or you run a lot of graphics?
I'm guessing you are asking the question because your MacBook is getting hotter now than when it was newer? That can happen due to a build up of dust internally but you've had it cleaned out by a technician, so you can probably rule that out.
As batteries age, they need more frequent charging, in the extreme, continuously! This is normal and a general recommendation is to replace a battery when its efficiency falls below about 60%. Batteries generate heat as they charge, so that may be what you are seeing. There are utilities that you can download from the web to monitor battery condition but I don't know if there are any for OS X. Apple may have one built in already - I don't know, I don't use a Mac.
Mac's are often used for their generally excellent graphics performance and that can be a source of heat too. I can run my Lenovo Thinkpad flat out CPU wise and while the base will get a little warm around the fan outlet, it is nothing compared to running a heavy graphics or video program - Youtube can make it too hot to touch near the exhaust but then a Thinkpad uses the same heatsink assembly for the CPU and GPU.
More mechanically, the Core 2 Duo processors were relatively power hungry, compared to today's Cor i's and so you could find the base getting a bit "cozy" because that power ends up as heat. The temperature of the fan exhaust should give a clue.
Another possibility is that the power connector plug has become a little worn with use and that can result in an increased electrical resistance, which again, will end up as heat. Give the connector plug a gentle shake, it should feel firm; if it feels loose, it may be getting a bit warm. A technician should be able to fit a new one to the end of the cable unless you are adept at soldering.
The earlier posters suggestions are good too, try running without the battery, just mains power. If that cools the system down, then it might be time to get the battery checked - a worn or poorly connected Li-Ion battery can fail pretty spectacularly - ask Boeing! Fan assisted cooling pads can help too, if a little inconveniently. If you decide to go this way, get one designed for the MacBook, so the fans are optimally placed.
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