I bought an off brand one on ebay and received it the other day and just quick hit the trigger just to check and make sure its working and all it does is buzz or hum is this normal for little canister sprayers to make this noise until it starts to pump paint or thinner or should I hear a pumping noise or air coming out right away?

Nope the buzzing noise is what you will hear. Is it a Wagner? When the paint or stain or whatever you are using primes into the head the buzzing will become different. The spray head you have will require an atomizer. They are typcially plastic and very small. You use different type atomizers for different types of materials say latex or stain.

Also be careful not to put your finger in front of the sprayer. There are all kinds of warnings about injection wounds and some can be fatal.

gold did it come with a manual? There are on the wagners a section on how to adjust for max preformance,besides that cheap little plastic atomizer,that can last 5 minutes or all day.If it stops spraying change the plastic atomizer first if it does'nt work then trouble shoot! I've also installed new atomizers that were bad and did'nt work at all.