Oh Kate, I feel for you. It was a bit different with Alex because he was almost 3 by the time Catherine was born but even then, he took her arrival very well and before long, it was like she'd always been here. Our only issue was that his potty training took a step back but I'm guessing you won't have started that yet so shouldn't have that problem.
Alex hadn't been away from me very much either as I'm also a SAHM but he took the 3 days he was at PIL's very well (I had to be in hospital for 48 hours after my ELCS) I expected him to be clingy and upset that I'd gone away fro so long and come back with a teeny screaching thing that needed all my attention but he just seemed interested in Catherine and was more interested in wanting to cuddle her than being upset. Yesterday he was suddling and kissing her loads so he obviously doesn't hold any grudges :)
We had books to read about new babies which maybe helped to prepare him. I know Noah is younger but he might understand more than you think. Our favourite was called "There's a House Inside me Mummy" for while I was pregnant. After she was born, The Baby seemed to sum up the frustration of having a new baby in the house perfectly. My friend's baby is due next week so she's borrowed our books at the moment else I'd post them to you. I think they're fairly cheap from Amazon though if you wanted to try them. We also got Catherine to bring Alex a present he really wanted when she arrived which immediately put her in his good books 
It'll be OK. My friends boys are 18 months apart and are so close now because of it. I don't remember the eldest being jealous when his brother was born (it's 12 years ago now!!) He was just too little and carried on oblivious.
I hope baby comes soon and comes easily.
I was supposed to be taking us all to a walk in the park today but I'm now absolutely convinced that my tonsillitis is back. Last weeks swab came back negative but when I went back on Tuesday, the nurse could see (sorry TMI) pus again which I didn't have before. I've got a temperature and pain in my ears whenever I swallow. I can't have any more ABs without a definite swab result as I've already had too many this year - from my last 3 lots of tonsillitis since March! - but I'm going to make a doctors appointment and see if I can have some better pain killers. Nurofen plus isn't halping at all this time
I thought I felt bad last week but I feel absolutely awful today. Just what I need for the holidays.