I really appreciate all your advice and suggestions! DH & I are probably the last persons on earth who don't have smartphones, so white noise apps won't be useful! But I'm sure we can find a radio at ILs and just tune in to static. OTOH, we may still get a white noise machine, but not sure if I have time to do that before we leave.
FWIW, last night we turned off our humidifier just to see. Baby M did fine during the first half of the night, but got very fussy after MOTN feed. I think it was b/c of gas (we weren't able to get him to burp) rather than the lack of white noise. He day time naps are in the living room and I don't have white noise on anyway.
He is usually most calm in the mornings so we'll try to do the car trip then. But since majority of the trip is on the Mass turnpike (with few exits and few service areas), we may have to stop before he wakes up/gets hungry just to feed him, or else we may be stuck with 30 miles of nonstop screaming!