As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: AIRCRAFT RETURNED TO LHR DUE TO A BURNING SMELL ACRID SMELL IN THE AIRCRAFT ALONG WITH A VIBRATION. AN EMERGENCY WAS DECLARED AND THE FLIGHT LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE DETERMINED THE CAUSE OF THE ODOR AND THE VIBRATION WAS DUE TO THE LEFT AIR CYCLE MACHINE SIEZING. REPLACED AIR CYCLE MACHINE LH IAW AMM 21-52-02.