Not sure if I have a problem or not.
My 2011 SD4 automatic had an annoying vibration at idle which was part vibration, part sound that reminded of the pressure waves you get when travelling at speed with the back windows down, a sort of wop wop noise. The noise/vibration was more noticeable when the engine was warm and held on the footbrake or handbrake.
This led me to this thread http://www.freel2.com/forum/topic13347.html about the missing drain valve on the airbox.
Mine was missing so I ordered the suggested rubber washer from EBay. When fitting I noticed that air was pulsing out of the airbox via the drain hole. Fitting the valve has considerably lessened the noise. If I listen at the vent in the wing I can hear the noise. I was expecting the airbox to suck air through the drain not blow out. Is this normal or is something wrong? Anyone fancy putting their hand under their airbox and see if its sucking or blowing?!

Water drains are normally in high pressure areas to force water (and some air) out. They normally have a 'bird mouth' on the end of the rubber tube to stop air being drawn up.

Following up on my own post. I took my Freelander to my local specialist. Apparently blowing and noise from the airbox is normal but at idle mine seemed noisier than normal. He had it in for the day and discovered that by playing with the revs and the angle of the car he could get the car in his words to 'bellow and play tunes'! The noise was coming from two areas. The exterior wing vent buzzing at certain revs. This was fixed with a bit of automotive tape between it and the wing. The second problem was the pipe within the wing. This wasn't secured to itself or the car properly, so was rattling around and also sucking and blowing at the joints, this was a wing off job to fix.
The car is much improved The noise is greatly reduced and the vibration when acceleration below 2000 revs has also been reduced. If you are getting a vibration/rumble/grumble when accelerating up to 2000 revs it's worth checking the pipe is secure. Apparently you can poke it with your fingers through a hole in the inner wing.

interesting & something else to be aware of. I had an odd "boomy" noise from the engine bay which I traced to a stripped thread on an airbox cover screw. It was enough to result in a big increase in noise which went away when the airbox lid was pressed down at that corner . Sorted by mending the thread, they do seem sensitive to anything loose in the inlet tract. 1978 Laverda Jota. Stolen...
1980 Honda CBX1000 6cyl.
1978 Laverda Montjuic.
2010 Yamaha MT01.