Bicycle Mechanics - Spoke tension on a commuter MTB

I've had this wheelset for at least 3 trouble-free years, with no problems whatsoever. This week, I mounted a rack on the back of my bike, and loaded up the panniers for the commute to work. I hit a lot of big bumps and a few sketchy drops off kerbs, and I noticed the rear wheel is suddenly making a bit of noise. So I checked the spoke tension last night, and they're all consistently a bit on the loose side. It could be that they've been that way a while, and I just never noticed, but that's a bit unlikely due to my mild OC behaviour. Does that make any sense? It's a decent hub, laced with DB spokes to a Mavic wheel.

My guess is that they ALL aren't loose, but a lot are. You may have a flat spot on your rim.

Give your wheel a spin and use your brake pad as a focus point to see if your rim has a flat spot which means your wheel is not longer perfectly round.

It is especially hard to feel this on a mountain bike, but on a road bike with a severe case, you can notice it on long rides on a good rode; whumpa bumpa whumpa bumpa.

The non-drive side spokes should feel loose compared to the drive side spokes. Its very hard to tell if they are "too loose" without putting a tension meter on there and measuring the tension. Take it to an LBS with a tension meter and have them take a look at it.
Is the wheel straight? If so, don't worry about it.

I haven't thrown it on the truing stand, but it seems pretty straight. The noise concerns me though. I'll check it before my next ride. I didn't spend too much time checking every spoke, as the missus was calling me for dinner.