Long-time rack owner here; about 20 years and on 3 different cars. The humming is caused by a "standing wave" determined basically by two things: vehicle speed (really, air speed), and cross bar spread. The hum of the wind is like a plucked guitar string: the vibrating wind is equivalent to the plucked string, even though you don't see the wind, and the length of the guitar string is equivalent to the cross bar spread. Different lengths result in different pitch of the hum.
You'll likely notice the hum at a particular speed, and if you go significantly slower or faster than that, then the hum likely goes away. The trivial case is when vehicle speed is 0 and wind speed is 0: no noise. I'm not trying to be a smart-alec here, just trying to relate the physics of it. If, for example, you hear the hum at 35 MPH, then it's likely you will not hear the hum at 120MPH. (Likely - it all depends on the aerodynamic factors like where the rack is related to the front windshield.)
Here's the physics of it (standing wave, harmonic oscillation):
. The "nodes" are the cross bars.
You can do at least 3 things to resolve this. Test one at a time, to isolate the specific causes:
- change the bar spread; farther apart, or closer together; or
- move the front bar further forward, or further back on the roof; or
- disrupt the standing wave by adding accessories to the bar (a fairing or rack or light mounts, depends on what you want on your bar).
You may have to try all three, but I'm confident you can solve this.
One dedicated accessory for this is what Yakima calls their Yakima WindJammer. At $25 the WindJammer might be the easiest and quickest fix, but it won't fit on every type of cross bar. You might need 2 of them, one on each bar. Rotate it up/down to get the desired effect.
Yakima WindJammer, $25: WindJammer | Yakima
Once you have the correct setup you can leave the bars permanently mounted on your rig. That's what I've done, for some 20 years, with no humming (initially there was some, though, until I fixed it using the above). Personally, I think repeatedly mounting and removing the rack when you need it is a pain. If I had to do that, the rack would likely live on my garage floor and never be handy when I needed it.
One other related thing about this: on flatbed trucks hauling large boxes strapped down by come-along straps, the loader often puts a half twist on the strap. This helps keep the strap from vibrating like a guitar string by making it LESS aerodynamic. Similar concept.
Anyway, apologies for such a lengthy post, and good luck!