It’s been cold here…my 1998 V70 started making engine noises.
They sound like typical valve train noises. Misadjusted, or sticking
valves, or tappets. Noise was the most pronounced at idle. It would get
less noticeable as RPMs went up. Car still drives ok, CEL is not on,
there are no error codes. For grins I unplugged the EVAP purge valve
after reading few things about it. Restarted the engine, and to my surprise
all the noise was gone. Car sounded perfect…for few days. Then the noise
came back. Can EVAP purge valve cause this kind of behavior?

 

This sounds similar to the problem I am having (2002 S40). Did you ever find the solution. I am guessing altenator bearings, or idler/tensioner pulleys.

 

My V70 developed a ticking like what you are describing and it ended up being the altinator bearings. New altinator fixed the problem. You can us a stethoscope to pin point the ticking, Or put the handle of a screwdriver to your ear and the tip on the suspected parts and that can help identify the problem. If it is real cold it could just be your valves sticking like you said. If it was a altinator bearing or other bearings the noise usualy will increase with RPM's though.??? Keep us posted.

 

The sound doesnt increase upon acceleration. At idle its loudest (800 rpm) - however, if if I apply a little gas (1500 rpm) the sound does go away. After 5 minutes, when the car is warmer, it goes away. I have to assume its some sort of hydraulic fluid warming up. Think this still applies to the altenator?