What was found
WHILE CLIMBING OUT OF FL290, A LOUD RUMBLING NOISE CAME FROM UNDER PILOTS FEET THAT LASTED FOR ABOUT 10 SEC. UPON REACHING FL350, CABIN ALT WAS RISING AND DIFF PRESS WAS DECREASING. COULD NOT MAINTAIN A CABIN ALT BELOW 10,000 FT. DESCENDED TO FL240 CABIN ALT 8700 FT DIFF PRESS OF 5.0 PSI. FOUND DUCT IN RT WHEEL WELL TO NASI SYSTEM COUPLING LOOSE. SECURED AND SAFETIED COUPLING. LEAK AND OPS CHECKS GOOD.