Thumping sound resumes but changes

    The weird thumping or pumping sound has resumed. But its cadence has changed. For example yesterday we first heard it at 6:20pm and it sounded like it always does with a series of thumps that last between 5 and 10 ceconds. Then at 6:51 pm it repeated. This is not what we have been hearing for weeks now. Its usually every hour, on the minute; this was only 31 minutes between. Then at 7:21 we heard a single thump, followed by a regular cadence, very loud at 7:51. Again at 8:20 only one thump with the norm at 8:51. Nothing was heard at 9:20 or 21, but the normal pattern repeated softly at 9:51. 10:21 sounded one thump and a normal pattern at 10:51. Silence again reigned at 11:20 and 21 but the sound resumed, loudly this time, at 11:51. Today has been pretty much the same. Still only heard in the Library. Still not identified. Still driving me nuts.