Never been to the Lemp myself but I've done enough site investigations to know that "basement feeling" people keep describing at these old American mansions is almost always a combination of infrasound and low oxygen levels in sealed spaces. That said, I don't entirely dismiss it either because some of the accounts from that particular location are oddly consistent across people who've never spoken to each other.
What exactly did you feel? Pressure in the chest, sense of being watched, actual temperature drop? The specifics matter more than the general "it was weird" description. Too many people on here are happy to say something felt off and leave it there which is completely useless for anyone trying to build a proper picture.
Anyone else been down there? Genuinely curious if the accounts cluster around a specific part of the basement or if its spread throughout.