I have a Horizon Omega 2 which has a deck squeak when running and sometimes (but less so) when walking. I've used silicon on the shocks, lubricant on the deck. The belt is okay as far as I know (doesn't slip). Its less than a year old. Its not a motor related squeak as I can hear it when the treadmill is switched off just by bouncing the deck. I've also checked all the fastening bolts underneath - all are fine. And I've used lubricant on the support struts (its a folding treadmill).
The only thing I haven't done - mainly because I'm too scared ! - is remove the hood covering the motor.