1 Lucky Texan

probably start with some vacuum gauge testing. The ECU thinks the A:F ratio is way off, a vacuum leak could cause lean-running and possibly affect the evap system enough to throw codes. (gas cap seals well ?) Inspect the evap canister and all those parts/hoses.

do you 'feel' it or just hear the sound?

if the rear hubs have also been submerged - maybe a wheel bearing is going out?

possibly fuel starved - maybe check fuel pump's output. I think the 440 code is sometimes associated with rusted-out tank filler tube?

examine the brake pads - inner and outer - for 2 issues; are they very close to the same thickness?, and are they worn at an angle, wedge shaped? I'd also look at the edge of the rotor and areas of the caliper near the rotor and backing plate, etc. look for fresh metal/worn areas. you might also consider taking some temp measurements of the hubs with an infrared thermometer after some highway runs. See if one side is consistently 30-40 degs or more warmer than the other side.

the MAP sensor may look something like this; look for it plugged into the intake mani - sorta behind the alternator under the 2 spark plug wires at the coil. I think (just based on googling some images)

I'd be tempted to clear those and see what comes back. if voltage is actually wonky - that can cause 'spurious' codes to set. so, thoroughly check the battery and charging circuit. also, I'd imagine it would be easy to miss some ground connections with an engine swap so - google up some images and try to locate all the grounds and confirm they are good.

there are sensors at each wheel and a pump so, there very well could be other issues with the ABS. Since the car is new to you - it's worth checking the charging system too as low voltage will often show up in the ABS system first. Is the check engine light on? getting the codes and posting them here is often helpful. was that engine rebuilt with multilayer steel headgaskets? more details about the car/engine/swap and specifics about how it's running/driving poorly will help. There are guys here with tons more experience than me , I just read a lot, so, they can probably guide you better.

is it a 5 speed? many people report problems with synthetic oil in the manual trans so, stay with 'dino' oil. on the auto - the diff is completely separate and synthetic is OK. (Redline Lightweight Shockproof in our 03 H6 OBW helped cut diff noise/whine by about half)

that car likely has a MAP mushy brakes are most frequently from air in the lines so, strat with a good flush/bleed. many people complain of soobs having a spongy feeling when actually, the pedal travel is just a tad longer than other cars. But, if you still have a problem after a bleed, start by looking for stuck calipers or cracked caliper brackets or associated bolts. Then, if that's all good, it may be time to investigate new rubber or stainless steel lines at the wheels or a master cylinder brace.

if you're that deep - why not hone it and install new rings?

PS system may be pulling in air - several threads about that. The car can be run minus the TB covers, might be worth checking foe excessive belt movement and looking at the inside surface of the covers for a place the belt is hitting. Also, the 2 piece crank pulley might be separated from its rubber. Look for excessive wobbling/run-out and maybe use a Sharpie or some paint to mark a line across its front - run car, see if the line separates.

^^^ +1 to above inspect for broken lug studs/nuts. And loose axle nut. Also, the little Cardan/u-joint gizmo on the steering column can sometimes bind-up.

get a tensioner too. My 06 WRXes was very weak.

I would only use Gates or Mitsuboshi belts. If you replace the waterpump - I've read you should use a Subaru METAL gasket and a Subaru or Aisin pump. All but one of the GMB rollers I put in my WRX seemed as good as the original rollers. Of course, I dunno how long they will last. Nothing wrong with doing the next belt change at 80K or 90K instead of 105K if you're worried about the parts quality. Of course, you could also get OEM parts, but they are pricey.

there's the access plate for the other gauge sender - any other leak would be from a perforation of some kind I guess?

those have been know to drain batteries under certain circumstances so, watch for that.

caliper clearance would be the issue - someone else may know.

it takes 200-400 revolutions for any timing BELT marks to synchronize again after initial installation. Position the crank mark, then look at the cams. seems like the tensioner may have failed?

well, it's normal if you see the other wheel move. (the opposite direction if 'open' diff.) I like this video about differentials;

you might try boxer4racing.com , they used to keep some legacy bits in stock. kartboy, mishimoto, perrin, cusco, etc. may have some stuff ebay might get you some clear corners - not sure. There are youtube videos and othe r info around to bake your headlights apart to paint or install HID projectors.(the retrofitsource.com ,etc.) custom rims would help, you could pull, sandblast, paint your calipers, some stuff made for Imprezas might be adapted to your car. some of the guys here and over at subaruoutback.org and elsewhere may post stuff they find at junkyards in the classifieds. You can also post WTB in the Forums' classifieds - some folks may have some stuff laying around....

I suppose you could have had a massive fluid leak. Be sure to check the fluid level while idling - some people forget that.

first item sounds like aftermarket axles - search,- well known problem. I suppose it's possible a worn original axle 'could' do it too.

consider pulling the front diff's drain plug to inspect for large pieces of metal. If you have a clean container - the diff oil can be reused but, if the oil is more than 2-3 years old, consider new oil.

well, you may discover something expensive or surprising, but you seem to be in-tune with getting the cars up to as high level of reliability they're capable of, and will save a little $$ as you do a few services yourself, so - that's good. Puts you way ahead of 75% of the car-owning public.