Monitors often make a significant "Thump" as the HT voltage comes on, and/or the automatic Degauss operates.
Unfortunately without hearing it, it's difficult to say if it's a real problem.
Did it happen just the once, or every time. If just the once, it can often be caused by the Degaussing thermistor failing - which is OKish, but means you won't be able to degauss again, and there will be a slow bulid-up of magnetism in the monitor - and eventually distirted colours.
If it happens every time, it's difficult to say what it may be - and it could presage the end of the monitor shortly.