It's the fans making the loud noise. If the system is processing too much it will get hot which will start the fans. Some games can cause this to happen along with the PS Store. Clean the vents in the front and back shoot towards the right side vent. Don't shoot into the blow out vent were the fan is directed, you'll just be shooting dust back into the system. If you have it in an isolated area, pull it out and put it in the open. If you have any saved data files, game downloads, or game installs that you don't need, delete them as it will give your PS3 more performance.
With the older models, it's good to have a little fan blowing behind it. It gave my old phatty an extra year or two of life, I'm sure of it.
The fan coming on is normal. It's even normal for it to be loud at times. If you just used your vacuum, I would suggest you get a can of duster which is compressed air. Shake it and shoot it in the vent holes on the front and back towards the blow out hole. This will push the dust out and towards the blow out vent. A vacuum can only suck from a certain distance til it has no suction to pull. You might be able to suck small particles, but you won't get the chunky ones unless you're up close to them.