Routine maintenance is essential as amplifiers do suck in a lot of air through the fans which will leave deposits inside the amplifier and also on the fans housing and over time this built up can cause the fan to catch as it goes round and cause excessive noise, compressed air is the best thing to use and will require the amplifier to be opened and the dust blown out, you may also need to clean the fan, earbuds are really good to use dipping them in nail varnish remover to clean in and around the housing. PS make sure it's not plugged in when attempting this