[SalesForce] Metadata API – Entity of type ‘CustomObject’ named ‘xxx’ not found

I'm trying to retrieve the metadata of all the objects. I retrieve all the objects using
soapConnection.describeGlobal().getSobjects()
and then insert them in my custom class Tables. This is then used in the Metadata class to obtain the metadata. However, I ran into this Warning message:

unpackaged/package.xml – Entity of type 'CustomObject' named 'CaseFeed' cannot be found
…
unpackaged/package.xml – Can't retrieve non-customizable CustomObject named: HashtagDefinition

The corresponding Metadata retrieval code is:

   public void retrieveZip(List<Table> tables) throws Exception
        {
            RetrieveRequest retrieveRequest = new RetrieveRequest();
            retrieveRequest.setApiVersion(API_VERSION);
            setPackage(retrieveRequest, tables);

            AsyncResult asyncResult = metadataConnection.retrieve(retrieveRequest);
            RetrieveResult result = waitForRetrieveCompletion(asyncResult);

            switch (result.getStatus())
            {
                case Succeeded:
                    // Print out any warning messages
                    StringBuilder stringBuilder = new StringBuilder();
                    if (result.getMessages() != null)
                    {
                        for (RetrieveMessage rm : result.getMessages())
                            stringBuilder.append(rm.getFileName() + " - " + rm.getProblem() + "\n");
                    }
                    if (stringBuilder.length() > 0)
                        LOGGER.warn("Retrieved warnings:\n" + stringBuilder);

                    LOGGER.info("Writing results to zip file");
                    File resultsFile = new File(ZIP_FILE);
                    FileOutputStream os = new FileOutputStream(resultsFile);
                    try
                    {
                        os.write(result.getZipFile());
                    }
                    finally
                    {
                        os.close();
                    }
                    break;
                case Failed:
                    throw new Exception(result.getErrorStatusCode() + " msg: " + result.getErrorMessage());
                default:
                    break;
            }

        }

private void setPackage(RetrieveRequest request, List<Table> tables)
    {
        com.sforce.soap.metadata.Package p = null;
        List<PackageTypeMembers> listPackageTypes = new ArrayList<PackageTypeMembers>();
        List<String> members = new LinkedList<String>();
        for (Table tbl : tables)
            members.add(Utility.getsObjName(tbl.getName()));
        PackageTypeMembers packageTypes = new PackageTypeMembers();
        packageTypes.setName("CustomObject");
        packageTypes.setMembers(members.toArray(new String[members.size()]));
        listPackageTypes.add(packageTypes);
        p = new com.sforce.soap.metadata.Package();
        PackageTypeMembers[] packageTypesArray = new PackageTypeMembers[listPackageTypes.size()];
        p.setTypes(listPackageTypes.toArray(packageTypesArray));
        p.setVersion(API_VERSION + "");
        request.setUnpackaged(p);
    }

Is there any way to retrieve the objects or do I have to resort to storing describeSObject() result for reference? The main reason is to make comparison between metadata during a period of time for detecting changes but I am not able to retrieve the metadata for a certain set of objects.