02Audi-A4-3.0

After driving around I'll come to low speeds to park and start getting this low-pitched knock sound. Sounds like something to do with the engine, but you can only notice it in the car (not with hood open). The noise is not constant and I can only hear it driving less than 5 mph and when idle.
I have had the car almost a year and it has 83,000 miles on the 3.0V6.
Just got 5w30 put in and coolant topped off at dealership.

I noticed a little smoke and funky smell one night from the coolant dripping on the hot cat converter, which is right under the coolant tank on the V6. probably just a drop or two and not much smoke at all.

I always let engine warm up before getting on it, and let engine cool after longer drives.

-Can a cat converter have a this thumping noise?
-Any loose or clogged engine engine hoses make a noise?
-Could it be a coil about to go?

I haven't noticed any performance loss. Thanks for input...