A.C.
I almost wrote this same question recently. Little baby Miles (10 months) SCREECHES too, especially if he doesn't get his way (minor things- drops a toy from the stroller, I take away something he can't chew on, etc). It's way toooo loud and it makes me crazy sometimes- I just can't think with that volume. He started at about 9 months, and I was worried that maybe I was 'spoiling' him, or setting him up for tantrums later; but I think you're right that baby Patrick is experimenting with his vocal capabilities and the great reaction they get - from what I've read, it is just a phase. Just be patient, and try to have a sense of humor- I don't think there's much you can do.
I try as best as I can to divert his attention, and I offer him respectful explanations without just 'giving in' ('Mommy's keys are sharp, so you can't chew on them', or 'Oww! Screaming hurts mommy's ears!') but he doesn't comprehend and screams anyway...one day he will surprise you and understand, but until they learn to communicate better, you might be stuck with the screeching for a little while.
I'm not sure how long this phase lasts since Miles' hasn't subsided (terrible two's?) but unless baby Patrick seems like he's in pain or he's acting sick, I'd say it's typical.
I cover and uncover his mouth to make different patterns while he's screaming, so instead of 'aaaaaaaa!' it's 'wa-wa-wa-wa-wa-wa!' and he thinks that's pretty funny and gets distracted. Turn it into a game and he might lower the volume and forget he was even upset.
Good luck, mama... hang in there!
-A.