Our newly acquired SP-15 has a very loud motor. Not sure if this is normal. We have a second SP-15 that is also loud, but not as bad as the newer one. I sometimes have to shout for my students to hear me. I received several suggestions to check/add oil, and we did that, but nothing changed. Is this normally a noisy motor? Is excessive noise a sign of something wrong? If it is normal, is there a way to dampen the noise, perhaps by adding insulation to the chamber that houses the motor, if that can be done without interfering with cooling? Thanks.