Guidry spent the 2022 season at Marshall and took the Tulane job last month.
Tulane is owed $500,000 in buyout money for Guidry's departure, a source told Thamel, and the payment is due to the school in the next 60 days.
Meanwhile, Miami defensive coordinator Kevin Steele has agreed to a deal to become Alabama's defensive coordinator, sources told ESPN's Chris Low on Sunday. This will be Steele's third different stint at Alabama under Nick Saban.
Under Guidry, Marshall's defense was ranked in the top five in six FBS categories: third-down defense (first), stop rate (second), passing defense efficiency (third), yards per play (third), rush defense (fifth) and turnovers gained (fifth). Marshall also finished sixth in scoring defense and interceptions, eighth in total defense and ninth in first-down defense.
Guidry had replaced Chris Hampton, who accepted a job with Oregon as the school's co-defensive coordinator and safeties coach.