Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 28; Flight Crew Total 2750; Flight Crew Type 950
Communication Breakdown; Distraction; Workload; Time Pressure; Troubleshooting
ATC Issue All Types; Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy
General Declared Emergency
Aircraft; Procedure
At 5;000 feet in cruise above a fog layer with tops at 4;000 and bases at 1;800 on an IFR Flight Plan (TER) from ZZZ to ZZZ1; a loud; continual thumping noise began to come from the front of the aircraft. I declared an emergency with ATC and descended without clearance. ATC restricted my descent to 2;800 feet for the airport Class [D] airspace at my 12 O'clock. I needed to get visual to allow for more options in the event I was forced to make an off airport landing; but ATC refused lower. I descended through the ATC restricted level to 1;800 feet where I became visual with the airport and landed safely.I am under the impression that once I declared an emergency and it was recognized by ATC; that I could proceed in whatever manner necessary to survive the emergency with no restrictions. The Controller and the Supervisor both asserted that their rules superseded my emergency and that I was in violation of the altitude restriction.  The Controller also gave me a deviation vector for traffic which I refused to accept and he then vectored the opposing IFR traffic away from my flight path.  I am unable to find any regulation supporting the Controller or his Supervisor's actions.
Pilot declared emergency and ATC had issues with what pilot needed to do to get aircraft on the ground as soon as possible.