Hello, and thanks for your question!!
It sounds like the fan bearing is going bad. When cold it is tight. When it starts getting hot it will loosen up and make the noise your hearing.
Or, the blade itself could be getting loose and doing the same thing.
I hope this answers your question. If so please click the green accept button. If not im still here to assist you.
is there any way I can get in there to repair or tie it up myself? Or this is something I have to have technician to repair?
To be quite honest, I would have a repairman doit. I dont like to see people try to repair things like heaters that can cause harm if not done right.
If you do decide to do it your self, I can try and help you with that. But I would need toe mod # XXXXX I could try to look it up on line. I need to see a picture of it.