The thunk is the relay that appliance modules use to control their loads. All of the lower priced appliance modules and non-dimming switches will thunk.
To avoid the thunk you have to step up a level. PCS makes the only plug-in "thunk-less" modules that I know of. It's the MM2F/S-15 which has two channels so it's like two appliance modules in one unit. It also gives you scene capabilities and all the other features of the PCS SmartSwitches. The PCS SSF/S is their wall switch for non-dimming loads, such as flourescent, ceiling fans, etc. and it's "thunk-less".
You can read more about the PCS SmartSwitch line at
www.pcslighting.com You can buy the products from
www.homeautomationnet.com I see that they don't have the MM2F/S listed but they can get it.