Can I reduce the grinding and humming noise from my portable air con's compressor?
Check to make sure one of the (presumably plastic) panels that make up the exterior of the unit are not loose. My unit had a "fancy" black plastic border that fastened around the front panel, and it would over time begin to work loose as the compressor vibrated. I ended up wedging a little folded piece of paper between the front panel and the bordering plastic to get it to shut up.

Other than that, put something vibration-absorbing under it if possible, as Ed Beal suggests. There's not a lot to be done. These units are noisy because the usual outdoor bits are now in a box next to you. But if you need cool air, you need cool air!