3GPP TS45.005 p.42:
For a BTS on a RACH or PRACH with a random RF input, the overall reception performance shall be such that less than 0,02 % of frames are assessed to be error free.
So random noise should be decoded as a noise for 99.98% of the time
0.02% of the noise could be understood as a request, by mistake.
There are 51 RACH-blocks in 240ms (approx)
so that’s 765,000 RACH blocks / hour
0.02% * 765,000 = 153 ghost rach per hour are “accepted” by 3GPP. In other words, if a BTS is located in an area without any traffic (anechoic room, for example), the number of ghost RACH self-generated should be less than 153 / hour.