I’ve heard SAC doing HF radio checks in the U.S.
SAC was dis-established in 1992, so it would have been prior to that time, and even if so— would not have been in clear-— maybe training band or something
Air Combat Command (ACC), Air Mobility Command (AMC), Pacific Air Forces (PACAF), United States Air Forces in Europe (USAFE) & Air Education and Training Command (AETC), United States Strategic Command (USSTRATCOM), which were under SAC at Offutt, were converted to Unified Combatant Command.
All of the above was transferred in turn to United States Strategic Command (USSTRATCOM) since 1992, also unlikely to send in clear— maybe to military ATCs or domestic in code.
In 2009 Air Force Global Strike Command (AFGSC) was established under USSTRATCOM, as command and control of all USAF bomber aircraft and the USAF strategic ICBM force. Would be very surprised if you verifiably heard them. Certainly hope not.