I use a Sundstrom SR500 belt-worn unit, with a SR540 headtop.
Despite the fan being worn on the belt there is a moderate amount of noise at the headtop if worn without ear protection. It doesn't bother me - but my hearing's not much good in any case.
The SR540 headtop is very comfortable but makes it impossible to wear ear defenders so I use expanding foam ear plugs, which do a reasonable job and also reduce the fan noise quite considerably.
However, it is generally considered that a decent set of ear defenders e.g. Peltor are more effective than ear plugs, although you do need to look at the type of ear defender and it's attenuation rating as well as the fit. Peltor Optime are pretty good, with three levels of attenuation available (Optime I/II/III) and headband/neckband/helmet mount options.
I find arco.co.uk a good start as they stock so many different types - useful for comparison purposes.
Duncan