Screaming/Screeching 6 mo: Is this normal?

[zebaby]

O.k. so I just can't figure out why he's doing this. My 6.5 mo DS has started practicing this "screechy" scream. At first I was mildly humored by it but sometimes it just grates on my nerves! He'll usually start with his usual vocalizations then change into this screech. He doesn't have a rhyme or reason for doing it (he's not in pain, doesn't seem to be frustrated about anything in particular, isn't necessarily trying to get my attention per se).

Here are a few other things he's been doing recently FYI in case any of you think they might be relevant: sitting up, pushing up & back with his arms while on his tummy (pre-crawl?), rocking while sitting, trying different solids (brown rice cereal, bananas, prunes, carrots, yellow squash, avocado, peaches, occasional suck on dinner roll), crying out in his sleep (for about 10-20 seconds; I can't explain that one either!)

If anyone has any insight into this new trick LMK. I'm a little concerned since DH's brother is severely autistic so part of me is paranoid that it can be an early sign. However, DS is really social and loves physical contact so he doesn't really fit the autistic mold, plus we have not vaxed at all, just to be on the 'safe' side.