From National Radio Systems Committee, DAB Subcommittee, “Evaluation of the IBiquity Digital Corporation IBOC System, Part 1 - FM IBOC,” November 29, 2001, p. 14
These illustrations clearly show how FM hybrid IBOC stations located near each other on the dial may interfere with each other.
In the first-adjacent channel interference scenario (upper), large swaths of each station’s spectral footprint overlap with another station.
In the second-adjacent channel scenario (lower), only the digital sidebands of the stations may overlap, which is less likely to have an impact on the analog signal but very likely to affect digital reception.
HD Radio interference on the FM band has been described as sounding like a "sharp buzzing," much like "a wood shop in high gear."