Rattlesnake Mtn is part of King County's "Big Backyard" series of  hikes.  There is a trail head at the northwest end of the mountain at Snoqualmie Point and at the southeast end at Rattlesnake Lake.  Walking from TH to TH is about 11 miles.  We took the RS Lake route which is 4.3 miles and gains ~2500 ft to the summit.