What it is doing:

- CEL code for O2 sensor out of spec (which went away after replaced coolant
and added conditioner)

- occasional CEL code for cyl 2 misfire (which I did hear once)

- had the "gurgle behind dash" which set all these events off (and what made
me do the coolant and conditioner thing)

- at 1800 RPM "brrapp" sound (probably one cylinder based on the frequency)
which sounds like a wood spoon on an aluminum pot) i.e. something is broken
or super worn in there (doesn't seem to be a heat shield)

- bubbles in the coolant overflow when the motor is running (now almost gone
due to the conditioner)

- loosing coolant to somewhere (probably down exaust pipe)

- "sweet" smell when stopped in traffic (this could be the cap not being
secured, I have looked at it a couple of times)

Other than the signs above, it does run OK.  I have not had it on long trips
in over a year and only use it to commute to work ~8 miles city driving.

I think the sound is probably an indication that it got warped due to very
slow coolant loss through the gasket causing a gas pocket to form and then
overheating.  Or it could be a broken valve or something like that.