Could be that since it has set for years without use or maintenance, the old oil has dried out and the motor is straining to run the machine without sufficient lubrication. Trying to run the machine in its present condition could damage the motor. Your machine very likely just needs a thorough oiling (1-2 drops of fresh good quality serger oil) every place that metal rubs on metal. (Sergers usually take a lighter-weight clear oil--different from what sewing machines would use.) You should probably throw out the oil that came with the machine. It is probably old and should not be used in your machine.
A serger requires much more frequent oiling (the moving parts behind the front door, ie the looper mechanics, etc.) than a sewing machine. In addition, about every year I take the bottom off my serger and lube the moving metal parts inside with Tri-Flow Synthetic Lube. It has a whole different sound after getting some TLC!
To keep your serger operating for years, be sure to regularly clean and apply a couple drops of lube as recommended in the owner's manual (maintenance section).