ill go get it.
Does the return duct size matter, as long as it's big enough? I'm just looking at the return duct here, that's whats making the wind through a crack noise.
The key operative phrase is "...as long as it's big enough...".
Oversized ducts are OK- low duct velocity reduces friction & noise. This is also true for supply ducts. Th only time oversized low-velocity ducts create a problem are when they run outside of conditioned space (such as those located above the insulation layer in an attic), made even worse if leaking and/or uninsulated. But that's a system efficiency problem (due to high parasitic losses/gains), not a noise problem.
High static pressure systems typically run 700-900 feet per minute duct velocity, but higher efficiency (and quieter) systems usually have a design target velocity in the 300-400 fpm range, though that applies primarily to ducts fully within the pressure & insulation boundary of the house (for efficiency reasons). A home efficiency & comfort company out in CA called Energy Docs made this handy nomograph for sizing duct systems for efficiency, including for low static pressure, low noise:
Do you have a model number on that air handler? (And condenser too, assuming a multi-speed air handler).
The opening size on the return is 24" x 24". A square duct that size would deliver a velocity of only 400 feet per minute @ 1200 cfm, which would normally be pretty quiet. Using the 400 cfm/ton rule of thumb for high static air handlers that duct would be pretty quiet with a 3 ton air conditioner, but could be singing a song or even screaming with a 4 or 6 ton system. (Being an engineer I hate using rules of thumb in lieu of real design, but many HVAC installers disagree.)
Not that it's going to be something you would change just to fix the noise, the odds are pretty good your existing system is way oversized for your actual cooling load (including the parasitic load.) An
HVAC consulting company in the Atlanta GA area made this graphic plotting the square feet of space per ton of load ratio on few dozen Manual-J load calculations against house size. Most of those were in the Gulf coast states. (It does not include parasitic loads of ducts outside of conditioned space, but those are generally no more than 20% of the total load.)
Rules-of-thumb designers typically use something like 700' per ton and spec the next size larger "just to be sure". For houses in the 2000-4000' range that's going to be 2x oversized, leading to lower comfort and sometimes lower efficiency and poor latent load handling. Ideally it would be no more than 1.2x oversized for the 1% design load, which would be sufficiently oversized to handle peak loads during heat waves, yet still deliver long comfortable cycles during normal hot weather.
Right sizing a replacement AC or heat pump onto an existing duct system designed for twice the volume yields lower duct velocities, less noise, and a higher, more comfortable duty cycle during hot weather.
To get a handle on your actual oversize factor it's useful to measure the duty cycle on the condenser on afternoons that dwell near the 1% outside design temp. It's amazing how often that turns out to be less than a 50% duty cycle (for single stage systems), which is an oversize factor >2x. Measuring the duty cycle of a multi-stage system becomes a more complicated, but is still possible by tracking amperage on the compressor across time, not just timing then on/off cycles.