Strange Noise

I have a 1997 Transport that had an engine rebuilt, original miles on engine were 80,000. My engine had gone out so now that I have a rebuilt one it's making a strange noise as if it were about to die out.

It feels as if it is not getting enough gas when it's idle. When I accelerate it does not make that noise and it runs fine. We have tried replacing the Mass Air Flow sensor since that's what the scanner had picked up but it did not help.

Do you have any ideas (everything from spark plugs, wires, water pump, belts, tensioners were replaced)

Thank you