A PLT WITH LITTLE CONFIDENCE IN HIS ACFT OVERSHOOTS HIS ALT WHEN AN UNUSUAL SOUND DISTRACTS HIM.

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A LITTLE BACKGND MAY BE HELPFUL TO THIS INCIDENT. MY PLANE WAS IN PORTLAND; ME; AND SVCED THERE FOR ENG PROBS. ON RETURN TO HOUSTON; THE PLANE WAS HELD UP IN KNOXVILLE; OKLAHOMA CITY; AND DALLAS FOR ADDITIONAL REPAIRS TO ENG AND AVIONICS. SOME OF THESE REPAIRS WERE REPEATS OF WORK DONE IN THE PRIOR CITIES. IT TOOK A MONTH TO GET THE PLANE TO HOUSTON AT GREAT TIME AND EXPENSE WITH INTERMEDIATE STOPS IN 3 CITIES. THIS SEQUENCE OF REPAIRS AND PROBS MADE ME LOSE CONFIDENCE IN THE INTEGRITY OF THE PLANE. I WAS NOT CONCERNED WITH FLYING IT OR AIRWORTHINESS; BUT APPREHENSIVE ABOUT WHAT INST OR SYS WILL FAIL NEXT. WHEN CLBING OUT OF DALLAS LOVE HEADED FOR HOUSTON; I WAS CLRED TO 4000 FT. A NEW SCREECH (IT SOUNDED LIKE AN ANIMAL BEHIND THE INST PANEL) BEGAN WHICH INTERFERED WITH COMS AND DISTR ME. I THOUGHT IT WAS A GYRO; BUT INDICATIONS WERE NORMAL AND THESE HAD JUST BEEN REPLACED TWICE. THE DISTR OF THIS NEW SOUND PLUS MY APPREHENSION ABOUT THE PLANE LED TO MISSING THE ALT RESTR. IT'S DIFFICULT TO ISOLATE CORRECTIVE ACTION. THE FIRST RULE OF FLYING IS 'FLY THE AIRPLANE.' I WAS FLYING THE PLANE; BUT THE SOUND WAS SO UNUSUAL AND NON MECHANICAL; IT DEMANDED MORE ATTN AND GOT IT. BEING APPREHENSIVE ABOUT AIRPLANE CONDITION AFTER A SERIES OF MECHANICAL PROBS IS BOTH NATURAL HUMAN BEHAVIOR AND SAFE PLTING. IN THIS CASE; IT LED TO AN ATC SEPARATION VIOLATION. I AM NOT SURE HOW I COULD HAVE AVOIDED THE INCIDENT. IT'S TOO SIMPLISTIC TO JUST SAY THAT THE PLT HAS TO HAVE AWARENESS OF ALL FLT CONDITIONS AND RESTRS AND THIS INCIDENT IS JUST ANOTHER TO HANDLE. I BELIEVE THE MORE IMPORTANT ISSUE IS ACFT CONDITION. THIS PLANE IS 24 YRS OLD AND THE SYS; EQUIP; WIRING; CABLES; ETC ARE A CONSTANT MAINT WORRY. DUE TO LOW VOLUME AND EXTENSIVE FAA REQUIREMENTS; UPGRADES ARE NOT AVAILABLE FOR THESE ACFT AT REASONABLE COST. IF NEW REPLACEMENT EQUIP COULD BE RETROFITTED AT REASONABLE COST; RELIABILITY WOULD IMPROVE AND THIS TYPE OF OCCURRENCE WOULD DIMINISH.