2005 Civic Manual 1.7l - Engine rattle when releasing throttle + white smoke from exhaust

[bhancock4 0]

I have a 2005 Honda Civic 1.7l manual transmission with 154,000 miles that does not get driven all that often.  2 years ago had major tune up...plugs, coils, timing belt, water pump, new clutch, etc.  About 2 weeks ago I replaced the upstream and downstream O2 sensors with Denso sensors because I had been getting a P0420 emission threshold efficiency code for about the last year.  That appeared to resolve the engine codes and has been running fine.

Fast-forward to yesterday and I noticed that when revving the engine beyond about 1,700 rpm or so and releasing the throttle it makes a horrible rattling/grinding noise from around the left side of the engine accompanied by a large puff of white smelly smoke from the exhaust...can't tell if the smell is "sweet"...smells bad to me.  You can see a video of me reproducing this sound here: Video of noise

The coolant was a little bit low (maybe 6-8oz) although I hadn't checked it in a while.  I did not see any milky residue under the oil filler cap or on the oil dipstick.  I did not visibly observe any bubbling under the radiator cap or the coolant reservoir (although the system wasn't full so maybe I would not have anyway).  I am purchasing a block test kit and will be doing that later today but this noise really concerns me.

Thanks in advance.

[bhancock4 0]

UPDATE:  I did the block test and it was negative.  Then used a long screwdriver to feel for vibration and it's definitely coming from the catalytic converter so I'll just swap that out.  Was hoping to get by with O2 swap, but I guess that's what I get for driving with that code on 🙂