Uh well, well, well... Maybe, make your tongue into the shape you would make an 'r' sound, lighten it a bit, and adjust pitch accordingly. I just do a Marvin the Martian impression and make it less enunciated and adjust the pitch, but it maybe different for you. If you want more tips, visit my Muppet Voice Impressions thread in the Classic Muppets section of the forum. You'll find lots of skilled people there, and you could ask them.
