I have a Johnson&Starly JB 40-50 Modairflow series heater, ?

My JB 40-50 Heater gives good heat, but has started making the most awful squeaking noise, if you give it a good bang, it stops for a while, but at 3 a.m. in the morning it is getting a bit much to get up and give it a big bang,also if we turn down the thermostat down it stops for a while but then the house remains cold!!!!Please can anyone help, I have had a heating friend in to look at it but he just shrugs his shoulders and said he thinks its the fan!