The crackling HEV Geiger-Counter sound effects appear to be missing from the radioactive area just after you kill the tentacle monster in Blast Pit (the very last section with the platforming on pipes before Power Up)
Most likely caused by missing trigger brushes to indicate the hazard.
I get the same thing just before blast pit as well (the area with the elevator in the middle of all that radioactive waste, where you transition between We’ve Got Hostiles and Blast Pit). If I’m a couple meters over the surface, I’ll get geiger counter sounds, but if I’m any closer to the surface the sound will stop. The noise shader still runs though.
Founded in 2004, Leakfree.org became one of the first online communities dedicated to Valve’s Source engine development. It is more famously known for the formation of Black Mesa: Source under the 'Leakfree Modification Team' handle in September 2004.