I'm rereading The Lord of the Rings. Appendix A, "Annals of the Kings and Rulers", tells us that Aragorn son of Arathorn spent part of his youth in Minas Tirith under the assumed name "Thorongil" serving under Ecthelion, Steward of Gondor.
Thorongil often warned Ecthelion not to put trust in Saruman the White in Isengard, but to welcome rather Gandalf the Grey.
Appendix B, "The Tale of Years", tells us,
2957-80 Aragorn undertakes his great journeys and errantries. As Thorongil he serves in disguise both Thengel of Rohan and Ecthelion II of Gondor.
10th July 3018 Gandalf imprisoned in Orthanc.
18th September 3018 Gandalf escapes from Orthanc in the early hours.
25th October 3018 Council of Elrond.
Saruman's treachery was not clear to anyone before the dispute with Gandalf in July 3018. And Aragorn did not learn of it till he and Gandalf met again in Rivendell in October. So why was Aragorn already suspicious of Saruman roughly 40 years earlier?