Noise #1 started out as a slight squeal on cold start when first under power. Just momentary. So I changed both belts thinking that was it. Well the squealing on first run from cold got much worse and louder and now lasts for half a block before quieting. It never happens on warm start only on cold. Doesn't happen on idle just when power is first applied. It isn't the power steering as the fluid is correct and all those hoses were replaced. I thought it might be the A/C so I turned that off but loud squeal is still there. It graduates into a sound that sounds like slipping then goes quiet and car runs normal. I put a new AGM battery in several months ago. A mechanic suggested maybe the battery is the problem and the alternator is trying to throw extra charge to the battery at cold start on first drive away. I have noticed on occasion it won't start on first crank. Changed the plugs and coils and that got better but still sometimes takes multiple cranks to start. Most of the time it starts right up.
So what could this be?

Noise #2 is a noise I've been hearing for years on this car (1995 XJ6). It sounds like a resonator when taking off from full stop and comes on at 1,500 rpm and goes away at 2,000 rpm. I feel a slight vibration when it happens. It's not loud or bothersome. It only happens under power. Does not happen when idling. Something is resonating. Anyone else have this?

 

That sounds suspiciously like a weak belt tensioner spring.
You could try an anti-slip compound on your belt. I have used treacle for years to stop slippage on rider mower belts and V belts on old Jags. Any sugar based syrup will do the job.

I recently watched the CAR WIZARD on you tube replace the centre carrier bearing on the tail shaft of a 1996 V12. The fault was similar to what you described.
Definitely worth a look

 

It might also be worth checking the harmonic balancer with some whiteout to make sure it's not de-laminated. I had a few startup and lower rev squeals when mine started to go.

 

Given what you have said about already rechecking the belt tension, this would be my thought too.
The X300 doesn’t use the spring type of belt tensioner btw.

 

#1 - I would bet on Nobeel's suggestion, the harmonic balancer. Common problem. Can be repaired. Not sure you can find a new part.

#2 - Not heard of that before... I don't have any suggestions.

 

Thanks but I replaced the Harmonic balancer with a new one about 3 years ago when I had a new cam chain put in.

 

Note: This squealing only occurs at cold start up in the morning. It lasts for about half a block then goes away.
All other start ups are quiet no squeal.
Could it be the new belts? Do they need dressing? Tension seems fine.
Squealing started next morning after new belts were installed day before.

 

1. Harmonic balancer. Noise will get worse and begin to happen at hot start-up too. Concur that the noise would start after replacing the alternator belt, as it will have better tension and traction on the pulley grooves, which is where the delamination happens. If your balancer is new and only 3 years old, could it be a substandard replacement item such as seen on eBay?

A very slight rotational adjustment of the camshaft position sensor "dizzy" could improve starting.

2. Timing chain noise. Although you replaced the chain and some parts, the AJ16 is known for timing chain noise which happens exactly as you describe and between 1500-2000 rpm. I can see it described as a resonance. It is generally harmless. It could be affected according to the type of oil you use.

 

If you are 100% sure your harmonic balancer is ok, ........The alternator belt does need to be pretty tight, you might just need to nip it up a bit more.