I managed to fix it yesterday.  Because of the nature of the fix, I don’t think the audio drivers have anything to do with the problem.  I don’t know why the problem is so much worse under Windows… I’m going to guess it has to do with the graphics card being more active when Windows is running.  But who knows?

Yesterday, I came across this page on Yahoo! Answers which hints at a fix, but doesn’t really tell you what it is.  Then I found these forums with a bunch of people complaining about the problem.  Inspiron 1520, 1521, and 1721 (and maybe 1525?) users also seem to have the problem.  More machines may have a similar problem as well.  Lots of threads seem to indicate that there is no solution, but finally, this one points out exactly what it is.  So, I went ahead and tried it myself, and it worked wonderfully.

Anyway, I decided to document the fix, so here it is with pictures.  Again, I have an Inspiron 1720, so these pictures are for that machine.  I believe the Vostro 1700 is pretty much the same.  Inspiron 15xx machines can be fixed in a similar manner, but there will be differences as to where the screws are and so forth.  To perform this fix, you’ll need a small Phillips screwdriver, a flat-head screwdriver or small knife, and some electrical tape.  Be warned, you have to pretty much take the computer all the way apart to get to the headphone jack on the inside, which is where we need to get.  The whole thing (disassembly, fix, and reassembly) took me a little less than an hour.  Note that I am not responsible if you somehow manage to break your computer or anything else while following these instructions.

In the pictures that follow, red arrows indicate something to be done, moved, or disconnected, and blue arrows point to screws that you need to remove.  And of course, your laptop is off and unplugged before you begin.

Start by flipping the machine over and removing the battery.  Easy enough.

Now, we need to remove the optical drive.  You only have to remove one screw holding it in, and then it slides right out.

(Oops, by the way, that paper you see taped to the bottom of my machine is there to keep it from wobbling, as it seems to be a bit uneven when it is on a flat table.  )

Now, to remove the hard drive(s).  Start by removing the panel covering them.  It comes right off after loosening these two screws.

After you remove the panel, some more screws need to come out to remove the hard drives.  While there are holes for four screws for each hard drive, my machine only had three screws for each, with one empty.  Of course, two hard drives are shown here but you may only have one.

After the screws are out, give each drive a tug on the little flap at the top and they should come out easily.

Now, to remove the keyboard.  First, we need to remove the panel above the keyboard.  Insert a flat-head screwdriver or knife into the slot at the right, and get it loose.

Now, you’ll have to move the flat-head screwdriver or knife along the bottom of this panel, unsnapping it as you go across.

Once you get all the way across to the left side, if the screen is tilted all the way back, the panel should come off easily.  Four more screws need to be removed, and then the keyboard can slide back and out.

Now, we’re going to remove the screen.  First, flip the machine back over and remove the wireless card cover.  Only two screws and it pops right off.

Unplug the little wires from each card.  These are the connections from the card to the antenna, which is in the screen.  I only have the “standard” wireless card, but you might also have a WWAN card to connect you to a cell network.

Now, some more screws holding the screen in.  There are two on the bottom and two on the back.

After those screws are out, flip the computer right-side up and open the screen again.  We need to disconnect some more cables.  Remove the grounding screw near the video cable connection, and then unplug that cable.  Also, unplug the Bluetooth card if you have one, and remove the card completely (it pops right out).

We also need to unplug the cable that connects the microphone array from the screen.

Now, carefully remove the screen.  You just lift it off.  You’ll have to navigate the cables we just unplugged out of their little holding paths, including the antenna cables that go through a little hole to the other side.  Be careful, there might be more antenna cables than the ones you unplugged.  I had three extra ones for the WWAN card that I do not have, and they were kind of hidden and stuck underneath.

That’s it, screen is out.

OK, we’re getting close.  Now, we have to remove the palm rest.  This one has the most screws.  Flip the computer upside down and remove, count ‘em, 16 screws holding the palm rest on.

Now, flip it back over and remove the last 6.

Now, you have to kind of wrestle the palm rest off of the computer.  Be careful, there’s still a cable going from the touchpad to the motherboard.  You shouldn’t need to disconnect it.  At this point, we’re just trying to expose the headphone and input jacks.

Here’s the culprit. There are metal tabs attached to the bottom of the palm rest, that make contact with the metal casings around the two audio jacks.  This allows for noise from elsewhere in the machine to make its way to your headphones.  I don’t really do any recording on my computer, but I bet that it would also add some noise to any audio input you have as well.

To compensate for this design flaw, we just need to put some electrical tape to separate the metal around the jacks from these metal tabs.  I actually put tape around the jack casings, like this…

…and also on the bottom of tabs themselves.

Update, September 22, 2009: Teejay in the comments below suggests not putting the tape on the jacks, just on the tabs.  While putting it on the jacks didn’t cause any problems for me, this is something to be careful about, and if you have audio problems after this procedure and you put tape on the jacks, maybe you should remove it and see if it is resolved.  (Still put tape on the bottom of the tabs.)

And, that’s all you need to do.  Now, follow these steps in reverse to put the computer back together.  When you turn it on, you can plug in your headphones and find them to be whisper quiet.  The background noise is gone.