Hi Haribabu,

One of the most important thing that can solve your problem hasn't been discussed yet. Here it is...

1. Do you use ZERO CROSS switching for your contactors if not then this is one of the major causes of your problem.

When zero cross switching is done than the cotactor contacts are opened or closed only when the AC cycle is going through the zero thus switching is done at effective very low voltage and produces far lesser noise than the random switching.

2. Also please specify how do you energise the 240V AC coil of your contactors. Do you use SSR? or Triacs? If triacs what's the stuff that fires your triacs? Answering this questions will give us more idea about your setup and will help us help you.

Regards,
Prahlad Purohit