Mining Bitcoin at 4TH/s

Hey, found a used ASIC miner for around £100 and it mines Bitcoin at 4TH/s. I don’t have to worry about electricity costs as I get that ‘Free’ (Private boarding school, electricity doesn’t cost me a penny) I was wondering if my payback time before I started making profits would be long and is there any way of keeping noise down at the same time. Also would there be any fire danger. I plan on running it in my dorm during the day and was curious if I could get noise level down to a level where i could sleep with it. Thanks!

Think I sorted noise cheaply, look at this bargain:

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