There are a few factors that help silence a model 99 quite a bit. Sewing regular fabrics on medium speed should not be noisy at all, top speeds will accelerate noise levels.
Check the oil points on top of the body with a tooth pick, they can be clogged up by decades of dirt and grime. Oil will not reach where it should go until it's cleaned out. It can get tedious to read it all the time but spending a bit of time cleaning and oiling everywere is a must. Don't forget the bobbin case and race, it makes a bit difference on this model.
The factor that's often forgotten is under the base of the machine. If it's fitted in a wooden case make sure there are felt under there. I have placed a lot of sturdy furniture felt under the base of mine and it helps keeping the machine stable while sewing. Place it on a sturdy table too if possible. Singer used to sell rubber mats for use under portable models, but a sturdy thick piece of felt does the same trick. I'm not sure what kind of feet my 99K had originally, there's only a screw left in each corner, but I suspect rubber or felt. It should not be a noisy model. I have the old version with the knob stitch length adjuster.