2000 V70XC SE AWD 167k miles. 2.4 LPT engine. Purchased it recently to fix up to help someone. Stumped on 1 remaining problem. Engine revs prefectly smooth at all RPMs in park. However, has a either slight miss or exhaust flutter (hard to distinguish) at 2000-2500 RPM and more prominent under load (like accelerating up a hill from stand still). All accessories off (no AC, ECC off) Here is what I've checked so far. Engine perfectly smooth below and above this RPM under load. VIDA DiCE reports no codes. There is an intermittent ECM 4801 code. Rear O2 was changed 20k miles ago. No record of front O2VIDA's ETM test shows max 2+ degrees of difference between TPS, well within Volvo's spec of 13+ degrees max tolerance. ETM is yellow labeled so been replaced at least once.Upper torque mount is new, lower transmission torque mount has no lateral play indicative of broken small bushing. Motor mount on driver+passenger side looks good (no collapse or crack). All rubber bushings (no poly)Rehabbed the PCV system. Bypassed the factory boost circuit with larger 5/8 ID hose (tapped it above the oil trap for intake manifold banjo nipple of course) Updated Banjo bolt to newer design with check valve. Blew air through all channels and components in the PCV circuit and engine block and all are clear. No oil leak anywhereGot an extra coil and rotated it through all 5 cylinders. This rules out any marginal coils. Replaced plugs and no change to above symptomGot an extra fuel injector and rotated it through all 5 cylinders. This rules out any marginal injectors.Replaced TCV (had one in hand) and all hoses to silicon. Tapped the turbo CBV connection on intake manifold to a pressure gauge. Good boost, can get 6+ PSI. Tested turbo CBV and opens under vacuum.Replaced all EVAP hoses to silicon. Replaced one blown check valve (closest to the intake)Replaced passenger axle, replaced front CV joint on propeller shaft, rotated back tires to frontFront tires are tight when off the ground. No play vertically or horizontally. All ball joints in the steering linkages are nice the tight.No obvious exhaust leak cracks or soot around exhaust manifold, turbo, flex down pipeUnplugged the exhaust CVVT solenoid, makes no difference. Haven't check the CVVT HUB for angular playSerpentine belt tensioner roller has a little angular play. Not enough to make noise. Only noticed during inspection while belt was offCompression test shows about 190 PSI across all 5 cylinders. Didn't do leak down (no tool)Car seems to be down about 3+ MPG (17) from the 2001 V70XC I owned previously (20) driving on same road+patterns (some hills, suburb, no highway). Maybe related to this flutter or need a front O2.New synthetic engine oil, PS fluid, drain+filled ATF, flush brake fluid, 20k mile old angle gear so new AG fluid in there (no leak)No leak anywhere in the engine (PCV, turbo oil lines etc...)I tend to think while the ECM 4801 code might point to a failing cat and/or front O2, engine revs smooth and RPM climbs quick above 2500 RPM so unlikely to have a plugged cat. Anyhow, seems like I've checked air, fuel, spark, turbo, compression, and mounts. What else is there to suspect? UPDATE: Had a second set of ears listen to this. There is no power loss so we thinks its not a miss. Its exhaust noise. We did find a small 3/4" leak on the output of the cat where it attaches to the pipe going to the mufflers. I'll start by plugging this leak. Pic attached.