Chicks go thru stages. Right now - your chicks are at the age they have realized they are FOOD and everyone loves chicken. So they run to save themselves. Loud noises, quick movement, shadows moving across the brooder - all things to trigger flight.
Move slowly - very slow. Leave your hand in the brooder until they come forward to check it out - do not move it. Chicks are also naturally attracted to natural nails, so show them your finger tips to entice them over. Talk gently to them.
I recently read that chickens process images much, much faster than the human eye. What we see as a blur - they see as regular motion. This "fast" sight allows them to see predators when they first start to rush forward. Slow is always better around chickens.