1996 Chevy S-10 Sounds like air sucking noise from engine,

Hi I have a 96 s-10 that has had a lot of work done lately. After all the work I have noticed (while driving) I can hear what sounds to me like a sucking air noise comming from the engine..? Also, when sitting at a red light or stop sign, once i press the gas 3 out of 4 times the truck wants to kindof stall and then it comes backs and accelerates.. It has 166,500 miles on the chassis but the motor was recently replaced with a used one that the shop claimed to only have 75,000 on it. This was done about 2,000 miles ago. As soon as i got the truck back from the shop I noticed the noise. They told me they can't do anything about it because it doesnt bring up a trouble code. I have recently Changed the sparks and wires and there is a new distributer cap also. Any ideas??