The motor has 171k on it, I had pretty much resigned myself to the idea that it's timing chain related (tensioner or guides).  It's not a quiet tick, it's pretty loud at idle.  I never notice it while driving unless I'm stopped.

Here's where I'm confused.  Yesterday I was stopped and had a barrier next to me.  I didn't hear it.  Next red light, I did.  I got home and tried seeing if it stops again.  I'd rev to about 1500rpm and the tick would speed up, but definitely got quieter.  Still sounding like timing chain, right?  Let off the gas, tick would slow, the moment it hit baseline idle it stopped ticking, then about 5-10 seconds later it would start again.  The fuck?  I repeated the same thing a few times and it behaved the same.  However, I did notice that after the brief lull in ticking, when it would resume ticking there was a very slight stumble.  The stumble reminds me of a small vacuum leak.

I don't know much about modern emissions stuff, but I'm starting to think this is a solenoid or diaphragm ticking away and that ever so slight stumble is it initially opening back up.

Thoughts/ideas of what to chase after?