Why are speed restrictions imposed for flights flying below 10,000 ft?
The speed limits were set by FAA in the early days of jet age for better control of the aircraft and safety where the air traffic (in VFR) is the densest and have little to do with noise. 

FAR § 91.117 states


  Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots (288 m.p.h.).


TWA Flight 553, a DC 9 collided with a Beechcraft Baron, killing 26 people on March 9, 1967. NTSB investigation determined that due to the high rate of descent of the DC-9, its pilots were not able to see the other plane in time to avoid a collision.

Following this accident the FAA issued Advisory Circular 90-32 titled 'Air traffic control and general operations, radar capabilities and limitations' in Aug 1967.

According to NTSB report on the accident, 


  The FAA has adopted a rule establishing that all aircraft flying below 10,000ft m.s.l will be limited to a maximum speed of 250kts effective December 15, 1967... (to) provide a more realistic 'see and avoid' environment in the airspace below 10,000 feet mean sea level (MSL) where the traffic congestion is greatest ....