light problem

I have a 600W HPS that I got brand new around a year and a half ago. The light is hooked up to a timer, but sometimes the light does not turn on unless I actually put my hand in the fixture and gently touch (sometimes a very slight turn) the blub with a cloth. There is always a low pitched humming sound coming from the ballast leading up to the point where I touch the bulb. Some days it will be on while others I will have to turn it on in the manner mentioned above. The bulb its self does not appear to have a darkened tip as with many ageing bulbs. Suggestions?

Sounds like a loose bulb connection to me,its kind of a catch 22 deal because you dont want to over tighten & break the bulb so many people do not tighten the bulb enough,ive done this myself twice,just take a clean rag & when the light is cool unscrew & reseat the bulb tighter.

Alot of times when the bulb is loose you can hear a slight rattle comming from the bulb when the bulb first starts to light up.