Sidney sits next to 4 phonetic neighbors — Sydney, Sidney, and Cydney are the closest matches by sound. Most share Sidney's English-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"from Saint Denis"
"cedar tree, resinous wood"
"selected, chosen one"
"creative elaboration of Finn"
"emperor, ruler"
"dark settlement, coal town"
"wealthy protector"
"hero, protector (fictional)"
"from the pool or marsh"
"thankful, grateful"
"son of Peter (son of the rock)"
"thunder ruler, powerful"
"little poet, little philosopher"
"son of Javan, modern invention"
"watchful, vigilant"
"one with a sacred name"
"loud revelry"
"father of peace"
"name of God"
"land power, territorial lord"
"dweller by the cliff ford"
Similar sound: These names share phonetic patterns with Sidney — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Anglo-Norman French origins, giving them the same cultural heritage as Sidney.
Rhythm: Sidney is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.