Our Trailblazer does the same thing & in the post below is just about exactly what I was told by the dealer

LINK


From the link..........
I have a 2005 Trailblazer and the mechanic said that the air pump engaging is the cause of the flickering lights. I went to the shop and had them manual turn this air pump on and it duplicated the problem. I have since been in contact with gm and they claim this is normal. Well if this is normal then the saleman should have explained this to me and then I could of made a decision to buy this Trailblazer with flickering light. If I had a choice to buy or not with this information of flickering lights I would not have purchased this Trailblazer. Headlight are to remain lite for safety not flicker and have a semi come into your lane. or a police officer pull you over and issue you a warning for flickering your head lights. Come on you think a 26,000+ dollar trailblazer should have flickering headlamps.