Ticking noise, increases with rev, compression dropped began not to long after rear tire changed from 190 to 180? friends say cct, turned to valve rings?

Many times, rear tires that are out of round or severely cupped or feathered, or that are out of balance can create this type of issue. My first recommendation would be to have them and the wheels on the rear of the car inspected. There may not seem like there is much wrong visibly if you look at just the tread of the tires, but a good tire tech can spin them up and give you some solutions, and the inspection itself is inexpensive. Hope this helps.

One thing you haven't mentioned is if the frequency of the ticking increases with engine revs? Or does it increase with road speed? Or stay the same no matter what? If the freq increases with revs the problem is most likely to be in the top end of the engine. if it goes up with road speed, it is likely to be in the region of the final drive system. If it stays the same it is likely to be an electrically driven ancillary. I hope this will (at least) point you in the right direction.

The noise is coming from the rear HVAC unit, one of the actuators for the blend door is trying to move but is unable. Either the blend door is stuck or the actuator is bad.

Trent I had this with another scooter and it turned out to be a cut or piece missing from one of the tires. Check all the tires carefully for any little defect or gouge. If you find something fishy take a piece of tape and cove the defect and the drive the scooter and see if you still hear the ticking sound. If that doesn't help block the rear of the scooter off the floor and have your wife run it slowly carefully listen to both rear tires to see if you can find the one making noise. If you can't hear the noise block up the front tires spin them to listen for the sound you are hearing. Hope this helps and let me know what you find Good Luck

1999 Isuzu Rodeo Service Repair Workshop Manual DOWNLOAD
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so it has some to do with rotation. you can jack one front wheel up, set parking brakes, start the car and put in drive. which ever side make noise, that's the one you need to inspect cv-axle, wheel bearing, ect.
check any thing that turns with the wheel

Hello,
Noise can be also from some sticky brake pad, or the bearings of the rear wheels, or even some spoiler, but you should have noticed it right after installing, and it is another sound. As for the differential, there might be a broken tooth, or again bearings, regardless of oil state.

rear bearings , Turn steering wheel side to side to side while driving that will load the bearing on that side see if the noise changes

Bad hub bearing. Drive the car on a straght EMPTY road .@30 mph or when noise is audible, **** the wheel left than right (similar to a race car warming. tires) and listen if there is a change in the sound if so its a front bearing if not it should be a rear bearing .Jack the rear of the vehicle up and spin the tire holding onto the strut. Feel and listen for a vibration or noise. This should pinpoint that elusive noise.Let me know Good hunting

i agree it could be,if you havent changed the oil and filter lately do that first using the correct oil grade if that fails let me know then we can go a step further