CEL with P0507 - vacuum leak?

I have an issue with CEL and code P0507, (idle higher than expected). I also notice a slight roughness to the engine at idle and up to about 2000rpm, above that it's smooth. Engine feels strong and pretty much normal. Idle does not seem high, it's just over 1000rpm as it has always been, but every once in a while, it doesn't settle down after coming off the throttle and it hangs at around 1400rpm. If I blip the throttle, it settles down to normal. If I clear the code, it comes back usually after 2 or 3 driving cycles (first pending, then confirmed). Over the winter I did the timing belts, cams out for new seals and I set the cam timing per the WSM. This is the 3rd time I've set cam timing on this car, the last time was 4 years ago, and after setting the timing, the marks on the cams lined up the same as they did before I took them out, so unless something has moved, I'm pretty sure cam timing is correct, or at least the same as has been for the past 4 years and 25,000 miles. I'm taking things apart to see if I've damaged a gasket or a hose somewhere. So far the only thing I've found is one single nut on the intake manifold (to the head) was loose, seems I forgot to tighten it, only threaded on a few threads. My question is, could a single loose nut cause a leak, or enough of a leak to cause a high idle? The intake is big piece and pretty rigid and the other 23 nuts are tight. Seems unlikely to me, but I'm happy to be wrong. I know... at this point, I might as well remove the intake and and cam covers to check again the cam timing, but since the high idle is not consistent, and I did check the cam marks probably half a dozen times before closing it up, it seems more like a leak somewhere to me.

 

The throttle cable is fairly new and the throttle bodies are opening and closing properly.