07 Sonata 2.4l Clunking

(This is copied and pasted from my thread in another forum dedicated for this car make, didn’t receive much help so I am now coming to you guys)

Hello! I own a 2007 Sonata 2.4l GLS with 120,500miles on it. In the past three weeks (recently after oil change) its developed a clunking noise only when warmed up (about 40 seconds). I purchased this car beginning of August and it had no noise then. I changed the oil with 10w30 castrol hm oil (the dealer I bought it from stated they put in same viscosity although oil cap reads 5w20 manual says 5w20-10w30 depending on climate is acceptable) . I’ve had it checked out by mechanics who have no clue what it is and want to tear down the engine. The noise goes away if it revs a bit higher that 1500rpm and then cones back worse when revving down until idle where its not as bad again. I’ve changed sparkplugs (they were factory anyways) and ran premium fuel and even bought a Hyundai filter afterwards hoping it would help but to no avail. I have a video sound clip where its clearly audible http://www.youtube.com/watch?v=ZVSjxpsOspU&feature=youtu.be

Your help is greatly appreciated

UPDATE- I’ve tried running an oil thickner from STP which claims to cushion noisy valves and engine noise but it did not help at all, infact the noise became worse. Today I poured in a whole can of seafoam into the crankcase before I change my oil and filter again on Thursday, should I try running a 5w30 Mobil 1 synthetic and if noise goes away try the HM version of the same viscosity synthetic? I believe the HM oil may have cause some debris to either come loose and stick the valves or removed the debris that was cushioning the noise.

EDIT: Could it be the PCV valve or loose flexplate bolts? I ordered a PCV valve which comes in Tuesday i’ll be replacing that since it is a maintenance item. I don’t to check the flexplate if it doesn’t sound remotely close to being the issue since I’d have to go through the hassle of removing mudguard, lifting car and removing the inspection cover.