1 Lucky Texan

is the car new to you? tires all the same? size/brand/model? they need to be, also near each other in wear. does the smoke from the exhaust smell like coolant? (sweet-ish, like toasted marshmallows.) or fuel or? is coolant level OK? ALWAYS check the radiator for actual coolant volume anytime there is even a hint of a problem. The overflow bottle does need to be maintained near it's normal level, but is a risky indicator of coolant volume. how does the trans fluid look? many folks have trans issues get better with fresh fluid. Doing 3 drain/fill cycles, with maybe a little driving in between, gets you around 85% new fluid. any dents in the trans pan? does the car car jerk/buck in tight circles on dery pavement in the same way regardless of FWD fuse use?

good job, it is weird how they fail. I had one on my WRX that 'wiggled' about 1.5/2mm - no more, but never made any noise. Another one, in the front, got VERY loud....... they have weird failure modes for certain.

is this car new to you? are all 4 tires exactly the same? same brand/model/size and near each other in wear? They need to be. if tires are the same, and, if you are saying your car DOES exhibit this; that is, turns smoothly in tight circles when stone cold, but, jerk-y/bucks/'holding back' in tight circles on dry pavement when warmed-up, that can often mean the center diff is bad.

is this car new to you? maybe a swapped, mis-matched rear diff? mark the tire bottoms with chalk, drive forward for 10-20 tire revs, recheck chalk positions for synchrony?

You know a lot more about these things than me but, I've always read that center carrier bearings can really howl.

paging @dburton97128

man - there are hovercraft posts in this forum going back to '04 !

you can also run one matched pair on opposite corners, and a different matched pair on the other corners. (unless the Impreza has LSD rear diff)

did you use the dash/groove on the 'tab' at the rear of the crank sprocket - positioned at about 12 o'clock/under the groove/dash on the 'boss' for the cps - for the belt install? some folks mistakenly use the triangle/arrow on the front of the sprocket. That IS TDC, but is never used for belt installation. The line on the 'tab' is used as that puts all the pistons at half-travel position to clear any valve interference.

in the old days, I sometimes had to stack 2 gauges to get an odd size.

exhaust smell like fuel? anyway, when you find the problem, if it seems like it still won't start, hold the pedal to the floor while cranking in case the cylinders are flooded. That signals the ECU to 'clear flood condition'.

ah, I missed that earlier

has the car been run offroad? anyway, look between the rad and a/c condenser for buildup of debris/grass/bugs etc. Outbacks sometimes have partial blockage in that area. soob thermostats have a large wax capsule. most typical thermostats will lead to problems. Also, the circuit thru the heater core is important to the cooling system, but seems like that may be OK.

is it better IMMEDIATELY after a battery disconnect reset? if so, that could point to a bad sensor. If not, the problem could be something physical; valve problem or injector problem or ???? the ECU ignores sensor readings for a coupla drive cycles after a reset, it just reverts to using a factory baseline ROM with nominal values. in theory - lol

make sure you look in the radiator itself - never trust the overflow bottle to reflect actual coolant level when there is even a HINT of cooling problems.

wonder if a light stream of propane fed in the intake while idling would make a change in the symptom? might exonerate or prove a fuel issue instead of spark???

NGK plugs?

how's the power? Knock sensors have failed, not set a CEL, but killed performance.

crazy! I have had to replace the lower section of the air box on both my cars, the 'slots' that hold the top's tabs had broken-out. sometimes when the top doesn;t get seated, it will make a noise.

there're FIVE of those b@st@rds on my WRX. I built my self a tool/spacer to use when I installed them. i THINK they should be 1mm above the belt but can shift when you tighten them so, you really need a spacer in there.

normal looking to me. Dunno about the really old trans, but newer ones find synthetic too slippery - many folks have issues with it, use plain, brand-name GL-5 or try the walmart synthetic blend GL-5. good value.

hmmmm.....some folks have found cracked oil pick-up tubes. =reduced oil flow. you can see/read horror stories at the KillerB site.

you checked the dipstick while idling right? and no confusion with the front diff stick on the other side? (right side)

belt likely intact (though, I wouldn't reuse it of course) the toothed idler probably puked its bearings or seized.

no oil on the plugs or plug wire boots? consider scanning ecu for pending codes.