noise from plant room

I bought my 1 bedroom apartment 18 months ago and have been happy here until about 3 weeks ago when I became aware of a low burring motor noise – it sounds a little like a muffled chain saw hum, goes for around 8 seconds, pauses and then goes again 24/7, but  there seem to be a couple of days a week that I can't hear it, which is making it difficult to track down.  My apartment sits above the plant room but I have not heard any noises at all from it until now.  About 3 days before I noticed the noise the boiler was replaced.  I don't know whether this is a coincidence or not.  The building manager has been helpful but as there is “so much going on down there” is at a loss as to what it could be.  My sleep has been disturbed as when I manage to get to sleep, the intermitent hum wakes me up again.  Have tried ear plugs and putting the radio on but cannot live with this.  Feel like I can't sell it or rent it out or live in it with this problem.  Help.  Sian