Salicylates cause hearing loss when applied topically. Again, this is not a proof that topical diclofenac cause hearing loss. But given the similarity of both substances, it is another reason to be very careful when using diclofenac on your head
Salicylic acid is a β-hydroxy acid. It may be used at concentrations of 20% to 30% for treatment of acne, rosacea, melasma, hyperpigmentation, mild photoaging, and rough skin. By acting as a lipophilic keratolytic agent, it produces exfoliation and dissolves follicular impactions, resulting in a reduction of acne lesions. Additional benefits of salicylic acid are antiinflammatory and antimicrobial properties. When applied, the acid crystallizes and may be visible after the first minute. After 3 to 5 minutes, the face is gently cleansed with tap water. Salicylic acid peels can potentially result in systemic toxicity, with the presence of nausea, hearing loss, tinnitus, and central nervous system dysfunction.
Source:
https://doi.org/10.1016/j.cps.2016.03.004
page 589
https://www.sciencedirect.com/topics/medicine-and-dentistry/glycolic-acid