To me noise like that looks like noise from belt. Point that it knows to go away, or stays then dissapear can be caused from damp weather which influence condition of the belt, so in damp weather it can start whining.
Can you say if at these times you had damp weather?

 

I have not had excessively damp weather those days, so I cannot confirm this theory.

Does it sound like a worn out belt to you? I would expect a whining or whistling in that case, not a tac-tac-tac sound, but I could be wrong for sure.

 

Can you tell if the noise is coming from the top or bottom of the engine?
It's difficult to tell but it almost sounds like what used to be known as tappet noise - but don't these engines have hydraulic cam followers etc..?

 

Can't tell if it comes from upper or lower area. I also couldn't figure out if it's internal from the engine or a belt area issue.

I also thought about tappets, that's why I checked oil level and also changed the oil after that. Don't know if they are hydraulic and I'm not sure what that implies either.

 

If in doubt, slaken off the alternator and remove the auxilary belt to check if the noise stops.

 

It doesn´t sound like a worn belt to me either.
It sounds a strange thing to do but believe me the following works.
Get a piece of flexible tube about a meter long. Put one end in or close to your ear. Move the other end around the engine to try to locate the source of the noise.
Knowing where it comes from will certainly help to diagnose what it is.

 

That sounds like a rough alternator bearing to me.

As has already been said, take the drive belt off and see if the noise disappears.

Be mindful that if the alternator seizes, the knock on effect from that could be quite dramatic due to the other parts in that area of the engine.

Good luck.

 

Agree. I recently installed a new alternator after getting a similar noise although mine was coupled with flickering lights on dashboard.

 

I think I've had something similar although I don't have a solution and I'm not very knowledgeable about engines so probably not much help. Similarly, when it happened it usually stopped shortly after starting up but not always and it has come and gone. I'm not sure mine was quite as loud as the videos, probably wouldn't hear it unless windows or roof was down. As far as I remember, it started after a cambelt+service. It didn't sound too bad and I'm low mileage so I ignored it. It was happening a year later so I mentioned it at the following service and it went away but I don't know what they did. It started again after I got my HGF fixed last year (mobile mechanic) but then I think it went away again on it's own. I checked it today and it didn't happen and I don't think it has for a while now.

 

From the description of Silver's noise I think this is most likely to be the timing belt cover. They aren’t enormously rigid when new and by the time they are a few years old they become likely to slightly touch against the cam pulleys, especially if any of the bolts are omitted when reassembling.

 

Thanks to everyone for your hints and ideas, they are really helpfull to me.
Since last week I did not hear that noise again, so maybe it has just disappear forever... (something tells me this is not the case).
In case it appears back I wil firstly check the alternator bearing, actually it does make a bit of sense since last owner replaced the alternator a few years ago and probably installed a cheap aftermarket one.

 

Very Similar noise that started while driving, after 150 miles and few days with suspicion of valve lifter problem since timing belt/water pump was changed before 1year / 6000 miles caused a major failure. Valve lifter broke down and destroyed the head / camshaft completely also timing belt jumped a tooth from that lifter that got stuck.

 

How did a valve lifter destroy the crank ? Would have expacted any damage to be limited to the head

 

Indeed, camshaft*

 

That sounds like tappets or a leaking exhaust manifold gasket.

The K-series does have hydraulic tappets and if the tappet's oil passage gets clogged with dirt/particulates, it keeps the tappet from getting enough oil and it will make that sound.