For quite a few years I was using the Real Bamboo Oil Diffuser Model 7521MJ made by Greenair Inc.  It worked perfectly until it started to make a sort of grinding noise.  At first, the noise only lasted a short while after turning the unit on but as time went on the noise lasted longer and finally it would not stop.  I called customer service and told the person who answered the phone about this problem.  Immediately they offered to send me a replacement after I let the customer service person hear the noise my unit was making.    I had discovered that the device needed to be cleaned from time to time and I had not done that at all and replied that I would like to clean the unit first.  My cleaning of the unit did help a bit but the grinding noise persisted.  In a couple of weeks, I was sent a replacement unit without asking for it.  I am very impressed with them doing this.  I am also impressed with the unit itself as it looks good and works very well.