4 possibilities, eg:
1. EGR pipe passing back of motor adjacent to firewall as @
threedogs mentioned. Try looking with mirror for soot/crack(difficult), would remove turbo heat shield.
2. Exhaust manifold gasket leaking (maybe broken studs_eg: common first 2 studs fail/ manifold warped), early stages, gap can take up after engine warms up and noise can stop.
Best to remove turbo heat shield and front exhaust manifold heat shield to get good visual. If you have manual boost control you can unwind your needle valve at idle and this will reduce boost level and sometimes the noise at early stages of failure can stop. When my exhaust manifold gasket failed, there was a metallic type honing squeal coming from around turbo inlet, thought it was turbo compressor wheel hitting against its housing bore. Changed out turbo and the noise was still present (with new turbo) and ended up being failure of exhaust manifold gasket (pulled down for the second time).
3. Turbo to dump pipe flange gasket due to nuts coming undone. Get under veh, look up under turbo heat shield and check for evidence of exhaust soot residue on shield inside face.
4. If only squeal (bearing like): Could be belt tensioner, at early stages of failure noise(squeal) can stop shortly after first starting engine. At idle try applying water at low pressure around tensioner and fan belt and see if noise changes.
Note: Have had all 4 failures since new (08 CRD) 150,000kms.
Suggest you buy a tensioner and new belt as you will need one anyway at some time (in case of failure_be prepared). With noise stopping since car service, most likely be tensioner(hopefully). Rather know what the problem is before travelling long distances over Easter.