[LAU] Rumble noise from mic

Hi all, When using the M-Audio Pulsar mic I notice a rumble sound until I touch the mic, in which case the rumble goes away. That sounds like a 'ground' problem of some sort. The mic is connected straight into a M-Audio 1010 PCI card using one of the XLRs jacks. The 1010 card also has M-Audio active speakers connected to it. The computer plugs into an electrical extension to which monitors and other devices are plugged. That extension goes to the wall plug and is of the three prong type (North America). Now, it could be that the house electrical outlet wiring is not right. Would that be the problem ? Is there any other way to solve this ? Gating the signal would hide the rumble when there's silence but that's not a solution. Thanks for any suggestions/hints/ideas. Cheers.