for some things, and some 19 month olds, probably a timeout is great (i think i was using them before my son was 2 also, once i knew that he understood the concept), but i do have a suggestion because it sounds like it's not working and well, we'll try other things when something isn't working, right? so what i'd do is look him in the eye and simply say "no screaming", firmly with a serious face. and then ignore him and it until he's done. i'd even walk away from him (when possible) when it starts. give him the impression you don't like it, or him when he's doing it. change your expression to deep disappointment and just walk away. it sounds like he's liking the reaction at this point, like you said. #1, don't give him any attention from it, and #2 make sure he understands mommy doesn't like it and won't stick around to hear it. tell him you'll talk to him when he's ready to talk like a big boy. (by the way this has worked great with my 2 1/2 year old son on whining and also throwing tantrums) good luck!
(PS, like some other girls said, at this age it's totally natural for them to test out new noises, over and over and over again -but the "Jump" you give him when he does it is reward aplenty, so i'm thinking he's past that "oh this is a new sound, let's try it out" phase, and is on to "watch what mommy does when i do this!" phase - you're going to have to try not to react - mamas need nerves of steel! might as well start training yourself now...soon he'll be "suprising" you with gifts of worms and bugs- or whatever else he can find to make you squeal!)