'Tik' 'Tik' noise for Vitz 2006 Engine

[asimaleem]

My vitz 2006 model (1.0L 3-cylinder engine 83K Kms done) is generating 'Tik' 'Tik' noise for the past two weeks from the engine bay when accelerator is pressed. This sound is noticeable at lower speed but goes away at higher speed.I got it checked from a local mechanic and according to him its coming from engine tappets and could be cam or bush or some other engine issue as well , which can be diagnosed only once engine is opened.
Then I took my car to Toyota 3s and their preliminary diagnosis was that it's tappets sound from the engine due to faulty VVT controller that's not letting the oil to flow freely at low speed, however as the car speed increases, flow gets bit normal and thus no audible tappet sound. According to Toyota 3S ,this can only be confirmed once they open the tappets cover and do the detailed analysis. If required VVT controller will be replaced and tappets adjustment will be done.

Both Toyota 3s and local mechanic suggested me to use 20W50 oil instead of 5W30 (Toyota recommended) and noise may go away .So I replaced the oil and tapped noise did go away 90% for 2- 3 days but has re-appeared

Questions.
1). Is Toyota's preliminary diagnosis correct... Any experience?
2). Is it safe/recommended to open the tappets cover and do detailed analysis let's say by removing something from engine.Can it cause some major screw-up? or should I let it alone as is?
3). Is it safe to get VVT controller replaced without any subsequent impact on the engine.Any risks?. Is it a common procedure?

Comments and suggestions please???